Embryology, the study of the mode of formation and development of the animal foetus. The progress of our knowledge on this subject has been marked by several well defined epochs, corresponding with the successive discoveries of as many different investigators. Though many important facts bearing upon embryology were known to the earlier anatomists and physiologists, they were often misinterpreted, and their true relations consequently mistaken. Aristotle and his followers recognized three different modes of generation as occurring among animals, viz.: oviparous, viviparous, and spontaneous generation. Oviparous generation was that form in which the female parent produced eggs, from which the young were hatched, as in most fishes, reptiles, and birds; viviparous generation was that in Which the young were discharged alive and fully formed from the body of the parent, as in quadrupeds and the human species; while spontaneous or equivocal generation was that in which certain animals of a low order, such as worms, insects, parasites, maggots, etc, were supposed to be produced spontaneously, without parents, from the soil, the water, or decaying animal and vegetable substances.

By the progress of investigation, however, the last mode of generation was shown to be much less frequent in its occurrence than Aristotle had supposed. The first advance in this direction was made in the latter part of the 17th century, when Redi, an Italian naturalist, studied with care the generation and metamorphoses of insects, showing that many worms and maggots, instead of being produced without parents, were in reality hatched from eggs laid by perfect insects, and that they afterward became developed by the process of growth into forms similar to their parents, He also in 1684 showed that most parasitic animals were provided with sexual organs, and produced their young in the same manner with other and larger species. Vallis-nieri soon afterward (1700) extended the observations of Redi, and applied the same conclusions to other species of insects, and to the parasites inhabiting vegetables. In this way the number of species in which spontaneous generation was regarded as possible or probable gradually diminished, as zoological science became more extended and more accurate; until, in 1837, Schultze demonstrated, by his experiments upon the infusoria, that even these microscopic animalcules are never produced in situations where their germs neither existed before nor could gain access from without.

Subsequently it was generally acknowledged by physiologists that spontaneous generation was a thing unknown in nature, and that the supposed instances of its occurrence were only cases in which the real process of generation had not been sufficiently investigated. The discussion on this point has again been taken up since 1858, and it has been maintained by Pouchet, Bastian, and others, that spontaneous generation may and does take place in the case of the lowest and most imperfectly known forms of infusoria, such as vibrio, bacterium, monas, and spirillum. Their experiments, however, are by no means regarded as conclusive by physiologists in general, a large proportion of whom consider the apparent uncertainty of origin as dependent on our imperfect acquaintance with the natural history and development of these minute forms of infusorial life. - The distinction between oviparous and viviparous animals was supposed by the ancients to indicate a fundamental difference in their mode of generation. In oviparous animals the eggs were known to be produced by the female and fecundated by the male, after which the young were hatched from them by incubation.

In the viviparous species the embryo was thought to be produced by a mixture of the male sperm with the fluids of the female generative organs; some thinking that the material for the body of the embryo was supplied by the menstrual blood, others that it came from a kind of female sperm, or seminal fluid secreted by the female organs. In 1651 Dr. William Harvey, in his book on generation, first announced the fact that there is no essential difference in the mode of generation between oviparous and viviparous animals, but that "all animals whatsoever, even the viviparous, and man himself not excepted, are produced from ova." But though the truth of this opinion has since been amply confirmed, and its expression (omne animal ex ovo) has now passed into a physiological aphorism, yet it was not intended by Harvey precisely in the sense which is now given to it. Harvey never saw the unimpregnated eggs of the quadrupeds, nor did he have any idea of the real structure and function of the ovaries in these animals; and in stating the opinion that the young of the vivipara and of man were produced from eggs, he only meant to say that after sexual intercourse and conception, the first thing produced in the uterus was not the embryo, but rather resembled an egg; and that the embryo was afterward formed from this by the process of growth.

In 1672 Regneer de Graaf showed that the ovaries, in women and in female quadrupeds, were filled with globular vesicles, visible to the eye, similar in appearance to the eggs of birds and fishes. These vesicles he pronounced to be eggs; and the organs in which they were found then took the name of ovaries. A century and a half later (1827) Karl Ernst von Baer discovered by the microscope the real egg of the human female and of the viviparous animals, which is contained in the interior of the vesicles of De Graaf. These eggs were shown to exist in the ovaries of virgin females, as well as of those in whom sexual intercourse had taken place; and it was accordingly demonstrated that in all animals and in man the eggs are formed originally in the ovaries of the female, independently of the male, and that these eggs are afterward fecundated and developed into embryos. Another important discovery remained to complete our knowledge on this part of the subject, viz., that of the spontaneous ripening and discharge of the eggs, in quadrupeds and in man.

Negrier (1840), Pouchet (1842), and Bischoff (1843) demonstrated that the eggs of the female, originally produced in the ovaries, ripen and are discharged, independently of sexual intercourse, at certain regular periods; and that the impregnation of these eggs by the male sperm is a subsequent process, taking place after the eggs have left the ovary and entered the Fallopian tubes. The origin of the embryo accordingly takes place in the same manner in all classes of animals, viz.: from an egg, which is produced in the ovary of the female, discharged thence at certain definite periods, and afterward fecundated by contact with the spermatic fluid of the male; and the only real difference between oviparous and viviparous animals is that in the former species (ovipara) the fecundated egg is discharged from the body of the female and deposited in the nest, or other suitable receptacle, in which it is afterward hatched; while in the latter (vivipara) it is retained in the body of the female, and there nourished during the development of the embyro.

The egg, at the time of its discharge from the ovary, consists of a globular vitellus or yolk, surrounded by a membrane termed the vitelline membrane, and containing a spherical vesicle termed the "germinative vesicle," marked with the "germinative spot." In very many instances this becomes surrounded, while passing downward through the Fallopian tubes or ducts, with a layer of transparent albuminous matter; as for example, in the eggs of frogs, tritons, etc. In other cases, in addition to the albuminous matter, certain membranous coverings are deposited round the egg, of a fibrous and calcareous texture, as in birds and the scaly reptiles. In all instances, however, it is the vitellus which is the essential part of the egg, and that from which the embryo is directly produced. - The first change which occurs after the impregnation of the egg is a spontaneous division or segmentation of the vitellus. The vitellus divides successively into smaller and smaller portions, in such a way as to produce at last a multitude of minute flattened bodies or cells, which are attached edge to edge, and which form accordingly a continuous membrane, which is called the blastodermic membrane.

In eggs which have a large-sized yolk, as those of the birds, lizards, and turtles, the formation of the blastodermic membrane begins at a particular spot on the surface of the vitellus, termed the cicatricula, and thence spreads in every direction, so as to enclose gradually all the rest of the yolk. But in those which are of minute size, as in quadrupeds and the human species, the whole vitellus is converted into the blastodermic membrane, which after its formation encloses only a small cavity filled with transparent, watery fluid. The blastodermic membrane then becomes variously altered and developed in different parts so as to form the various organs and tissues of the embryo. A line or furrow first shows itself in the thickest and most condensed portion, known as the primitive trace. This indicates the future situation of the spinal column; and the different parts of the vertebra gradually grow around it, forming a chain of cartilaginous rings, with transverse and oblique processes, which envelope the primitive trace or furrow, and convert it into a closed canal, large and rounded at the anterior extremity, or head, but narrow and pointed at the posterior extremity, or tail. In this canal the brain and spinal cord are formed and complete the development of their various parts.

At the same time the remainder of the blastodermic membrane becomes more condensed and organized, forming the integument and muscles of the chest and abdomen; and these portions finally unite with each other in front, forming at the point of junction a longitudinal or rounded cicatrix, known as the umbilicus. The alimentary canal, formed in the interior of the abdominal cavity, is at first entirely closed; but two openings are afterward formed, one at the anterior extremity of the body, the other at the posterior. These openings become respectively the mouth and anus. In frogs, tritons, and some kinds of fish, all these changes take place after the eggs are discharged from the body of the female. In birds and turtles the segmentation of the vitellus and the formation of the blastodermic membrane are already far advanced at the time the eggs are laid. In the lizards, most serpents, and some kinds of cartilaginous fishes, the development of the embryo takes place partly while the egg is still in the generative passages of the female, and partly after its expulsion.

In a few species of serpents, and in some fishes, the embryo is completely developed within the egg in the body of the female, so that the young are finally brought forth alive; while in all the warm-blooded quadrupeds, as well as in the human species, the fecundated egg is also retained in the uterus until the embryo is sufficiently developed to be born alive. - In the frog, the eggs are deposited in the early spring, in some shallow pool, freely exposed to the light and air. Immediately after their expulsion the albuminous matter with which they are surrounded absorbs water and swells up into a tremulous and gelatinous mass, which floats near the surface, with the eggs imbedded in its substance. The formation of the embryo then goes on as above described, and the young animal, at first curled up in the interior of the vitelline membrane, soon ruptures it and effects its escape. The body is at this time of an elongated form, terminating behind in a narrow, compressed tail. The integument is covered with vibrating cilia, which produce a constant current of fresh water over the surface of the body. Respiration is performed by gills, situated at the sides of the neck, which are at first exposed, but afterward become covered by a fold of integument.

The muscular system is very feeble, and the young animal remains nearly motionless, attached by the mouth to the gelatinous matter around the eggs, upon which he feeds for several days. As he increases in size and becomes stronger, he abandons the spawn and swims about freely in the water, feeding upon the juices and tissues of aquatic vegetables.

Mammalian Egg, highly magnified

Mammalian Egg, highly magnified. a. Vitelline membrane. b. Vitellus. c. Germinative vesicle. d. Germinative spot. In the left-hand figure the egg is shown burst open by the rupture of the vitelline membrane and the vitellus partly escaping.

The cilia with which the body was covered disappear. The alimentary canal is at this time very long in proportion to the size of the whole body, being coiled up in the abdomen in a spiral form. During the summer lungs are developed in the interior, and the young tadpole frequently comes to the surface to take in air. But the gills also continue in existence, and are still the most active organs of respiration. Toward the end of the season anterior and posterior extremities or limbs begin to grow; the posterior sprouting externally from each side, in the neighborhood of the anus; the anterior remaining concealed under the integument, just below the situation of the gills. The tadpole passes the winter in this transition state. The next spring the lungs increase in size, and the gills become less active as organs of respiration. The anterior extremities are liberated from their confinement by a rupture of the integument which covered them, and both anterior and posterior limbs grow rapidly in size and strength. The tadpole at this time, therefore, has both fore and hind legs and a tail. The tail early in the summer becomes atrophied, and finally withers and disappears altogether; while the limbs, and especially the hind legs, grow to a larger size.

At the same time, the lungs attaining their full development and the gills finally disappearing, the tadpole is thus converted into a perfect frog, capable of living and moving upon the land as well as in the water. The tadpole swims by the tail and breathes by gills, while the frog swims by the legs and breathes by lungs. Simultaneously with these changes, the alimentary canal becomes very much shorter in proportion to the rest of the body, and the frog becomes carnivorous in its habits, living principally upon insects, which he is enabled to capture by the great development of his muscular system, and the rapidity and suddenness of his movements. - The process of development of the embryo consists, accordingly, in the successive formation and disappearance of different organs which are adapted to different modes of life. When these changes take place after the young embryo has left the egg, as in the case of the frog, and so produce marked alterations in the external form of the body, they are termed transformations or metamorphoses. Thus the egg of the butterfly, when first hatched, produces a caterpillar, or larva; an animal with a worm-like body, sluggish crawling movements, and no sexual apparatus, but furnished with largely developed digestive organs and a voracious appetite.

This condition is succeeded by the pupa state, in which the animal changes its skin, losing the legs and bristles which were his locomotory organs, and becomes motionless and nearly insensible to external impressions, and stops feeding altogether. During this period another integument grows underneath the old one, with new legs and wings; and when the skin is again changed, the animal appears as a perfect insect, or imago, capable of rapid and sustained flight, ornamented with brilliant colors, provided with different sensory and digestive organs and a well developed sexual apparatus. - In those instances where the hatching of the egg is a longer process, similar changes to the above take place while the embryo is still retained in its interior. At the same time certain other organs are formed in addition, which either disappear before the time of hatching, or are thrown off when the young animal leaves the egg. With turtles, for example, the eggs, consisting of the vitel-lus, albumen, and shell, are deposited in an excavation in the earth or sand, and allowed to hatch in these situations. With birds, they are placed usually in nests, formed of twigs, leaves, and fibres, and there kept constantly warmed and protected by contact with the body of the female parent.

This process is termed incubation, and may be imitated artificially by keeping the eggs at a temperature of 104° F. and providing for the regular supply of fresh air and a proper regulation of the atmospheric moisture. During incubation the eggs of the common fowl lose 12 per cent. of their weight, of which 11 per cent. is due to the exhalation of moisture. They also absorb oxygen and exhale carbonic acid. The segmentation of the vitellus and formation of the blastodermic membrane, and of the organs of the embryo, take place for the most part according to the plan already described, but variations present themselves which make the process more complicated. The vitellus, for example, instead of being entirely surrounded by the abdominal walls, is divided into two portions by a constriction situated about its middle. One of these portions remains outside the abdomen of the embryo, though still connected with it by a narrow neck, and by blood vessels which ramify upon its surface. This sac, containing a portion of the vitellus, is called the umbilical vesicle.

It supplies the embryo with nourishment during the whole period of incubation; for immediately after the egg is laid the albumen, which is at first gelatinous in consistency, begins to liquefy near the upper surface, and the liquefied portions arc immediately absorbed into the yolk. The yolk, therefore, grows larger and more fluid than before, while the albumen diminishes in quantity, and loses its watery portions. The blood vessels of the embryo, ramifying over the surface of the vitellus and the umbilical vesicle, in their turn absorb the nutritious fluids from it, and convey them into the interior of the body, to be used in the formation of the tissues. At the end of incubation the albumen has disappeared and the umbilical vesicle has much diminished in size, while the body of the chick has increased at the expense of both; but the umbilical vesicle, containing the remains of the yolk, still exists, and is enclosed within the abdominal walls when the chick leaves the egg. In quadrupeds and the human species the umbilical vesicle is much smaller in proportion to the body, and less important in function, than in birds and the scaly reptiles.

In the human embryo the umbilical vesicle, always very small, diminishes rapidly soon after the end of the third month, and is hardly distinguishable during the latter period of gestation. In the egg of the fowl certain accessory membranes or envelopes begin to grow around the embryo at an early period. The first of these is the amnion, which is formed by a double fold of the blastodermic membrane, rising up about the edges of the body of the embryo, so as to surround it by a kind of circumvallation, or embankment. By continued growth these folds at last approach each other and meet over the back of the embryo, forming by their union and adhesion an enclosing membrane, or sac, which is the amnion. The amnion, therefore, is a membranous envelope, which is closed over the back of the embryo, but which remains open in front of the abdomen. About the same time a vascular, membranous diverticulum grows out from the alimentary canal, near its posterior extremity, and emerging from the open part of the abdomen turns upward over the back of the embryo, outside the amnion, and just inside the shell membranes.

This vascular membrane is the allantois, an organ which surrounds the embryo in quadrupeds, birds, and some reptiles, and which serves as an organ of nourishment, or for the aeration of the blood. It first makes its appearance in the early stages of embryonic life, as we have mentioned, in the form of a protrusion or offshoot from the lower part of the alimentary canal. It rapidly increases in extent, protrudes further from the abdomen, spreads laterally in every direction, and thus finally envelopes the body of the foetus; its edges, as they come in contact with each other from opposite directions, in many species becoming adherent, so as to form a continuous membrane. This membrane, which in the bird's egg is situated immediately underneath the shell, is abundantly supplied with blood vessels coming from the interior of the abdomen of the embryo, and returning thither after having ramified upon the surface of the allantois. It is by this means that the absorption of oxygen and exhalation of carbonic acid take place, which are so distinctly marked in the fowl's egg during the latter period of incubation. The allantois in these animals is accordingly an organ of respiration for the foetus.

Toward the latter period of incubation the allantois becomes very closely adherent to the egg shell, and the shell itself grows thinner, more porous, and more fragile; whence it is believed that the allantois also serves to absorb calcareous matter from the shell, which it conveys into the interior of the body, to be used in the formation of the bones, the ossification of which takes place about this period. When the chick is sufficiently developed to leave the egg, usually at the end of the 21st day, by a sudden movement it strikes its bill through the end of the attenuated and brittle egg shell, and by inhaling the air and continuing its struggles, finally extricates itself from the cavity of the shell, leaving the allantois adherent to its internal surface. The blood vessels of the allantois are torn off at the umbilicus, which afterward closes and unites by a permanent cicatrix. - Another important change which takes place in the development of birds and quadrupeds, in addition to those presented by frogs and fishes, is the formation of the urinary apparatus.

In fishes and ba-trachians the urinary organs are two long glandular bodies situated on each side the spinal column, which are known as the Wolffian bodies, and which remain permanent throughout the life of the animal, no true kidneys ever being produced. But in birds and quadrupeds the Wolffian bodies, which are at first very large and important organs, disappear during the progress of embryonic development, while the kidneys are formed at the same time, and gradually take their place as urinary organs. The kidneys are accordingly substituted for the Wolffian bodies in these instances, very much as lungs are substituted for gills in the development of the frog. - In many species of quadrupeds the allantois attains a large size, and performs a very important function, during intra-uterine life. In the ruminating animals, cows, sheep, goat, deer, etc, it forms an elongated sac, taking the form of the uterine cavity, and lying in close contact with the lining membrane of the uterus. The cavity of this sac communicates with the cavity of the posterior part of the intestine, from which it was originally developed, and receives the secretion of the Wolffian bodies, and afterward of the kidneys.

Its exterior is covered with a large number (60 to 80) of tufted vascular prominences, which are entangled with similar elevations of the uterine mucous membrane, called cotyledons; and the blood of the embryo, while circulating through these bodies, absorbs from the maternal vessels the materials requisite for its nutrition. In the pig the allantois is nearly smooth on its external surface, merely presenting transverse folds and ridges, which lie in contact with similar inequalities of the uterine mucous membrane. In the carnivorous animals its middle portion is shaggy and vascular, and entangled with the blood vessels of the uterus, while its two extremities are smooth and unattached. In the human embryo the amnion is formed in the same manner as already described; but the allantois, instead of constituting a hollow sac, with a cavity containing fluid and communicating with the intestine, spreads out into a continuous flattened membrane, the two layers of which are in contact with each other and adherent, leaving consequently no cavity between them. It extends, however, quite round the foetus, enveloping it in a continuous vascular membrane, which here takes the name of the chorion.

The chorion is, accordingly, the same thing in the human species as the allantois in the lower animals, except that its cavity is obliterated by the adhesion of its walls. It is covered uniformly at an early date with tufted villosities, which become entangled with the mucous membrane of the uterus. But during the third month it begins to grow smooth over the greater portion of its surface, while at a certain part the villous tufts grow more rapidly than before, until they are finally converted into a thick vascular, spongy, and velvety mass of villosities, which penetrate into the uterine mucous membrane, and become adherent to its blood vessels. This organ is then termed the placenta; and from that time forward it serves the foetus as an organ of absorption and nourishment, its blood vessels imbibing from the circulation of the mother the albuminous fluids which it requires for growth and nutrition. - The amnion in the human species is at an early period so arranged that it closely invests the body of the embryo, while between it and the chorion there is interposed a thick layer of soft gelatinous material.

During the second and third months the cavity of the amnion enlarges, by the accumulation of a watery and albuminous fluid (the amniotic) in its interior, while the gelatinous matter between it and the chorion is gradually absorbed and disappears, in order to make way for its expansion. By this enlargement the amnion approaches nearer the internal surface of the chorion, and by the beginning of the fifth month the two membranes come in contact with each other. By this means the foetus becomes enclosed in a large cavity (the amniotic cavity), filled with fluid, so that a free space is allowed for the movements of the foetal limbs. These movements begin to be perceived about the fifth month, at which time quickening is said to take place. They afterward become more strongly pronounced, and before birth are frequently very active. These movements are also favored by the formation and growth of the umbilical cord. The blood vessels of the foetus, termed the umbilical vessels, which pass out from the abdomen to the placenta and the chorion, become much elongated, and at the same time covered with a deposit of firm gelatinous matter, the whole being surrounded by a prolongation of the membrane of the amnion.

This bundle of vessels, covered with the above investments, is termed the umbilical cord. It grows very long, and also becomes spirally twisted upon its own axis, usually in a direction from right to left. There are in the latter periods of gestation two umbilical arteries, carrying the blood of the foetus outward to the placenta, and one umbilical vein, in which it is returned to the body and the internal venous system. - The formation of the blood and blood vessels in the embryo takes place at a very early period. Soon after the production of the blastodermic membrane, some of the cells of which it is composed break down, or separate from each other in such a manner as to leave irregular spaces, or canals, which inosculate with each other by frequent communications. These canals are destined afterward to become the blood vessels, the structure of which is gradually perfected by the growth of fibrous tissue in their walls, and their complete separation from the neighboring parts. In the interior of these canals, or imperfectly formed blood vessels, there is to be seen at first only a transparent, colorless fluid, holding in suspension a few large, roundish, nucleated cells, which move sluggishly to and fro, as the current of the circulating fluid begins to be established.

These cells do not differ much at this period from those which constitute the general mass of the neighboring tissues; but soon afterward they begin to be modified in their appearance, and give place to true blood globules, Their surface becomes smooth, and a reddish coloring matter is produced in their interior, which gives them a tinge similar to that of the red globules of the blood in the adult condition. The red blood globules of the foetus, however, still differ in several important particulars from those of the adult. They are considerably larger and more globular in shape, and have also a very distinct nucleus, which is wanting in the blood globules of the adult, at least in the quadrupeds. They increase in numbers also at this time by spontaneous division, one globule becoming divided into two, which separate from each other and afterward become themselves divided in a similar manner. In this way the quantity of the blood globules is very rapidly increased, and they soon become also still further altered in form and structure. They diminish in size, become in the human subject and the quadruped flattened and biconcave in form, and finally the nucleus disappears.

These changes are all effected during foetal life, and chiefly during the early months, so that at the time of birth the blood globules have already the characteristics which distinguish them in adult life. The multiplication of the blood globules by subdivision is a process which takes place only in the embryo. The perfectly formed blood globules increase in number in some other way, probably by the isolated production and growth of new cells. - At the time of birth the foetal membranes (amnion and chorion) are ruptured, and the foetus escapes. The umbilical cord being at the same time divided and tied, the portion still connected with the foetus soon shrivels and separates by spontaneous ulceration, while the spot at which it was attached heals in a few days, leaving a cicatrix on the middle of the abdomen, which is permanent throughout life, and which is called the umbilicus. - The limbs grow, by a kind of budding or sprouting process, from the side of the body. They are at first mere rounded eminences, without distinction of parts or articulations; but they subsequently become successively divided into fingers and toes, and the different joints of the arm and leg.

The upper extremities during the greater part of foetal life are larger than the lower, but afterward the lower extremities and the pelvis grow faster than the arms and shoulders, and finally become after birth much the larger of the two. The lungs are small and solid in texture before birth, but immediately afterward they expand by the inhalation of air, and receive a much larger supply of blood than before. On the other hand, the liver is much larger in proportion to the rest of the body at an early period than subsequently. In some animals it amounts during the first part of foetal life to 12 per cent. of the entire weight of the body, and is reduced to 3 or 4 per cent. at the time of birth. In the human subject it is equal at birth to 3 1/2 per cent. of the entire weight, but is reduced in the adult to less than 3 per cent. Great changes take place also during foetal life in the anatomy of the heart and circulatory system, as well as in the relative size and development of nearly all the organs in the body.

These changes continue to take place after birth, though less rapidly than before, and the entire process of development is not regarded as complete until the individual has reached the adult condition. - A very singular modification of the above process of embryonic development among the mammalia occurs in the marsupial animals, of which the American opossum (didelphys Virginiana) is a representative. In these animals the eggs are impregnated and the formation of the embryo commenced in the usual way; but after remaining for a comparatively short time in the uterus, and while their development is still very incomplete, the embryos are discharged from the generative passages, and are immediately afterward found attached by the mouth to the teats of the parent. They are then less than half an inch in length, and quite gelatinous and embryonic in appearance. They are protected by a double fold of the integument of the abdomen, which forms a kind of pouch, surrounding the teats, and serving to enclose the young and helpless embryos.

They remain in this situation during the completion of their development, continuing attached for the most part to the teats, from which they derive nourishment; and even after they have become capable of running about by themselves, they still, upon an alarm, take refuge for a time in the pouch as before. It is not known how the young embryos, when expelled from the uterus, find their way into the external pouch so as to reach the teats, for, notwithstanding many attempts have been made to ascertain this point, the animal is so secret in her habits at the time of delivery that they have been thus far entirely unsuccessful. - Among invertebrate animals the egg is constituted, as a general thing, in nearly the same way as in ver-tebrata, and its impregnation takes place also in a similar manner. The segmentation of the yolk goes on by repeated subdivisions, until the whole vitellus is converted into a mulberry-shaped mass out of which the embryo is formed.

While, however, in the vertebrate animals, the embryo always lies with its belly upon the surface of the yolk, in some of the invertebrates, as the articulata (insects, spiders, crustaceans), the back of the embryo is in contact with the yolk, and the closing up or union of the two sides of the body takes place along the dorsal line, instead of the abdominal. In many mol-lusks, as for example in snails, the embryo, soon after the commencement of its formation, begins to rotate slowly in the interior of the vitelline sac; and this rotation continues more or less rapid until the hatching of the egg. In the invertebrate classes the metamorphoses or transformations of the young animal are more frequent and more striking than in vertebrata. In many of them the young animal when first hatched from the egg is entirely unlike its parent in structure, external appearance, and habits of life. In the class of insects many of these transformations are well known, and have always attracted the attention of the curious.

Frequently the young animal, in passing through several successive transformations in which he is adapted to different modes of life, necessarily changes his habitation; and being found accordingly in totally different localities, and presenting at successive intervals corresponding differences of organization, the same embryo at different ages is often mistaken by the ignorant for an entirely distinct species of animal. These changes of habitation, occurring in the course of embryonic development, are termed migrations. They are often very marked in parasitic animals. Thus the taenia, or tapeworm, inhabiting the small intestines of certain animals, such as the dog, cat, etc, produces an egg containing a small globular embryo, armed with certain hard spikes, or curved prominences, capable of being moved by muscular fibres inserted into their base. The portion of the tapeworm in which these eggs are contained, known as the proglottis, is discharged from the intestine of the first animal, and the eggs, becoming mixed with vegetable matter, are devoured by animals belonging to other species, as for example the pig. Either in the process of mastication, or by the action of the digestive fluids of the stomach, the external envelope of the egg is destroyed, and the embryo set free.

By means of its movable projecting spines, the embryo then makes its way through the walls of the stomach or intestine into the neighboring organs, and often reaches distant parts of the body. Here, becoming arrested, it is temporarily fixed in place by the consolidation of the tissues round it, and becomes enlarged by the imbibition of fluid, assuming a vesicular form. A portion of this vesicle becomes inverted, and at the bottom of the inverted part a head is produced, upon which there are formed four muscular disks, or suckers, and a circle of calcareous spines or hooks, different from those present at an earlier period, which are thrown off and lost. In this state the animal receives the name of scolex or cysticercus. It remains in that condition till the death of the animal whose tissues it inhabits, when, being devoured with the flesh by an animal belonging to the first species, it passes into the intestine of the latter, and there becomes developed into the complete tapeworm, or strobila, similar to that from which its embryo was first produced. The same animal is accordingly a parasite in different organs, and even in different species, at different periods of its development.

Some of the invertebrata are parasitic at one stage of their existence, and lead an independent life at another. Such are the small Crustacea which infest the bodies and gills of certain fishes. In the family of oestridoe, or bot flies, the eggs are deposited by the female insect, and attached to the hairs of horses, cattle, etc.; from which situation, after the embryo has become partly developed, they are detached in some instances (as in gastro-philus equi) by licking, and swallowed into the stomach. Here the larva is set free, and attaches itself to the mucous membrane of the stomach, nourishing itself upon the fluids obtained from this source, and gradually increasing in size. After a certain period the larva lets go its hold, passes through the intestine, is discharged with the faeces, and, assuming the pupa state, is finally transformed into the perfect insect. The process of embryonic development is accordingly a succession of changes, in which the structure and organization of the young animal are adapted to different modes of existence, and in which different organs and apparatuses, successively appearing and disappearing, replace each other in the progress of growth, and give rise to the appearance of transformations, which affect the body as a whole. - See Harvey, Exercitationes Anatomicoe de Generatione Animalium (London, 1651; Sydenham edition, London, 1847); Spal-lanzani, Saggio di osservazioni microscopiche, etc. (Modena, 1767), Prodromo sopra le pro-duzioni animali (Modena, 1768), and other works; Baer, Be Ovi Mammalium et Hominis Genesi Eptistola (Leipsic, 1827), and Ueber En-twickelungsgeschichte der Thiere (1828); Valentin, Handbuch der Entwickelungsgeschichte des Menschen (Berlin, 1835); Coste, Recherches sur la generation des mammiferes (Paris, 1834), Embryogenie comparee (1837), and Histoire generale et particuliere du developpement des corps organises (1847, '49, '53); Pouchet, The-orie positive de la fecondation des mammiferes (1842), and Theorie positive de l'ovulation spon-tanee et de la fecondation des mammiferes et de l'espece humaine (1847); Bischotf, Rei-fung und Loslosung der Eier der Simgethiere und der Menschen (Giessen, 1844), and his treatises on the embryology of the hedgehog (1853), ape (1866), etc.; Rathke, Ueber die Entwickelung der Schildkroten (Brunswick, 1848), Flusskrets (1829), Columber Natrix (1839), Krokodille (1866), etc.; Agassiz, "Lectures on Comparative Embryology " (Boston, 1849); II. Baudrimont and Martin Saint-Ange, Du developpement du foetus (Paris, 1850); Bergmann and Leuckart, Vergleichende Ana-tomie und Physiologie (Stuttgart, 1852); Kol-licher, Entwickelungsgeschichte des Menschen und der hoheren Thieve (Leipsic, 1861); Ernst Haeckel, Generelle Morphologie der Organis-men, vol. ii., Die Wissenschaft von den entste-henden organischen Formen (Berlin, 1866).