The first changes in the ovum independent of impregnation consist in the shrinking of the yolk from the vitelline membrane, and the extrusion from it of certain granular bodies which lie between it and the vitelline membrane, and are called the polar globules. The germinal spot and germinal vesicle also disappear, and are said, by some observers, to form these polar globules.

Changes In The Ovum Subsequent To Impregnation 282Sections of the ovum of a rabbit, showing the formation of the blastodermic vesicle

Fig. 269. Sections of the ovum of a rabbit, showing the formation of the blastodermic vesicle.

(E. Van Beneden). a, b, c, d, are ova in successive stages of development. zp. Zona pellucida. ect. Ectomeres, or outer cells. ent. Entomeres, or inner cells.

After the union of the male and female elements, a new nucleus appears in the vitellus which forms what is called the segmentation sphere. This divides at first into two segments, then into four, eight, sixteen, and so on, until a large mass of cells occupies the yolk. To this condition the name of morula is given, from its supposed likeness to a mulberry. Fluid now collects among the cells, and separates some of them from the others, and they arrange themselves into an outer and inner layer, consisting of different kinds of cells. The inner cells finally become aggregated at one part of the ovum in contact with the outer cells. The ovum now receives the name of the blastodermic vesicle.

In the hen's egg the cleavage is confined to the cicatricula or blastoderm, and does not include the rest of the yolk. From the fact that the cleavage of the yolk is only partial, such an ovum receives the name of meroblastic. The human ovum, which undergoes complete segmentation, is called holoblastic.

The cells in the blastodermic vesicle become arranged into three definite layers, which are called respectively, from their position in the blastoderm, the epiblast, the mesoblast, and the hypoblast.

Transverse section of the medullary groove, and half the blastoderm of a chick of eighteen hours.

Fig. 270. Transverse section of the medullary groove, and half the blastoderm of a chick of eighteen hours. (Foster and Balfour).

A. Epiblast.

B. Mesoblast.

C. Hypoblast.

inf. Medullary fold. tnc. Medullary groove. ch. Notochord.

From these layers are developed the embryo and the membranes surrounding it, each layer being developed exclusively into certain tissues.

Thus from the epiblast, or outer layer, arise the epidermis of the skin, the brain and spinal cord, and certain parts of the organs of special sense; while it also aids in the formation of the chorion and the amnion. From the mesoblast are developed the skeleton, connective tissues, muscles, and nerves, in addition to the vascular system and the supporting tissue of the glands; one kind of tessellated cells arises from this layer, viz., the endothelium, forming the surface of all serous membranes. From the hypoblast spring the epithelial lining of the alimentary canal, that of the glands which are diverticula from it, and of the lungs; it also forms the lining membrane of the allantois and yolk sac.

The blastoderm of the hen's ovum, which is comparatively easily studied, consists of a small, clear, central portion, called the area pellucida, from which the body of the chick arises. Surrounding the area pellucida is a much larger zone, which appears less transparent; this, the area opaca, is devoted to the formation of the membranes.

Diagrammatic longitudinal section through the axis of an embryo chick.

Fig. 271. Diagrammatic longitudinal section through the axis of an embryo chick. {Foster and Balfour).

N. C. Neural canal. Ch. Notochord. D. Foregut. F. So. Somatopleure. F. Sp. Splanch-nopleure. Sp. Splanchnopleure forming the lower wall of the foregut. Ht. Heart. pp. Pleuroperitoneal cavity. Am. Amniotic fold. A. Epiblast. B. Mesoblast. C. Hypoblast.

The embryo is developed from the rest of the blastoderm in the following manner. At the front of the area pellucida a fold, or dipping in of the blastoderm takes place; this consists of a projecting part above and a groove below, and constitutes the cephalic, or head fold. The upper projecting portion of the fold tends to grow forward, while the groove grows gradually backward. Later on, another fold appears at the posterior part of the area pellucida; this is the tail fold. At the sides of the area pellucida folds appear, which tend to grow downward and inward so as to reach the under surface of the blastoderm and unite with the head and tail folds.

By the approximation of all these folds a canal is formed - the embryonal sac - which is closed above by the main portion of the area pellucida, in front by the head fold, behind by the tail fold, at the sides by the lateral folds, while below it is open to the vitellus. This canal ultimately becomes subdivided into an inner tube, the alimentary tract, and an outer one, which forms the body walls, the final place of union of the folds being marked by the umbilicus. It must be clearly understood that these primary folds which form the embryo include in their layers the epiblast, the whole thickness of the mesoblast, and the hypoblast, whereas the folds giving rise to the membranes do not comprehend all these layers.