This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2482). It is true there are no longer two vaginae terminating in a single cloacal cavity; but let the reader observe how nearly the vagina of the Rabbit (fig. 421, a, b) approximates the condition of a cloacal chamber. Anteriorly it receives the contents of the bladder (d, m); while the rectum (s) terminates by an anal orifice (r) so closely conjoined with the aperture of the vulva, that the anatomist is almost in doubt whether the external opening might not be described as common both to the vagina and intestine. Advancing from this lowest form of a placental uterine system, it is found that the two uteri before their termination become united so as to form a central portion common to both, called the body of the uterus, through the intervention of which they communicate with the vagina by a single passage, named the os tinae; still, however, the cornua uteri, especially in those tribes that are most remarkable for their fecundity, become during gestation far more capacious than the mesial portion of which they appear to be prolongations. It is, in fact, in the cornua that the numerous progeny of such animals are lodged during the whole time of their retention in the uterus; and consequently such an arrangement is absolutely requisite, as must be evident from simply inspecting the gravid uterus of a Sow (fig. 422), where the cornua uteri, (c c) are of remarkable dimensions.
Fig. 422. Uterus of the Sow.
(2483). As we ascend from the more prolific inferior races to the Quadrumana and the Human species, the proportionate size of the body of the uterus becomes materially increased, and that of the cornua diminishes in the same ratio, until in the Monkeys and in Woman the latter become quite lost, and the now pyriform central part appears to compose the entire viscus, into the cavity of which the Fallopian tubes seem immediately to discharge themselves. Thus gradually, therefore, does the oviparous sexual apparatus assume the viviparous type; and then, passing through numerous intermediate forms, ultimately attains its most concentrated condition in the uterus of the Human female.
(2484). In every other part of the generative system we shall likewise find the characters of the type at length completely established.
The ovaria (fig. 422, a) entirely lose all traces of their original racemose condition; for now the quantity of granular matter enclosed along with the germ in each Graafian vesicle, the last remnant of the yelk, has become almost inappreciable, and the little ovarian ovules are enclosed in a dense parenchymatous substance enveloped by a smooth albu-gineous tunic. The Fallopian tubes (b) correspond, in the smallness of their diameter, with the minuteness of the globules they are destined to convey from the ovaries into the uterine receptacle; and lastly, the excretory canal of the bladder (d) becomes quite separated from the vagina (e), and the anal and generative apertures are found completely distinct from each other.
(2485). After the above brief sketch of the anatomy of the organs of generation in the higher Mammalia, it now remains for us to trace the development of the germ from the moment of impregnation to the birth of the foetus, and observe in what particulars placental generation differs from the oviparous and ovoviviparous types already described. In the viviparous or placental Mammifer the effect of impregnation is the bursting of one or more of the Graafian vesicles, and the escape of the contained germs from the ovisacs wherein they were formed. In the Ovipara, owing to the delicacy of the ovisacs, the vascular membranes composing them, when once ruptured, are speedily removed by absorption; but in the Mammal this is not the case, and a cicatrix remains permanently visible upon the surface of the ovary, indicating where the rupture has occurred: such cicatrices are known by the name of corpora lutea.
(2486). On the rupture of the ovarian ovisac, the vesicle of PurJcinje, or the essential germ, accompanied only by a most minute quantity of granular fluid, or yelk, is taken up by the fimbriated extremity of the Fallopian tube, and conveyed into the interior of the uterus, where its development commences. Observations are wanting to teach us precisely what are the first appearances of the embryo; but there is not the least doubt that the materials for its earliest growth are absorbed in the cavity of the womb, and that its formation from a blastoderm, or germinal membrane, is exactly comparable to what occurs in the egg of the Bird, already minutely described in the last chapter (§ 2110 etseq.), and that, in every particular, as relates to the growth and functions of the vitelline or onvphalo-mesenteric as well as of the amniotic systems, the phenomena are the same as in the marsupial Mammal up to the period when the young Marsupian is prematurely born, to be afterwards nourished in the pouch of its mother from materials derived from the breast.
(2487). But precisely at that point of development where the Marsupial embryo is expelled from the uterus of its parent, namely, when the functions both of the vitellicle and of the allantoid apparatus become no longer efficient either for nutrition or respiration, a third system of organs is developed in the placental Mammifer, whereby a vascular intercommunication is established between the foetus and the uterine vessels of the mother, forming what has been named by human embryo-logists the placenta.
(2488). In the ovum of a Sheep, at that period of the growth of the foetus which nearly corresponds with the end of uterogestation in the prematurely-bom Kangaroo, all the three systems alluded to are coexistent and easily distinguishable, as will be seen in the accompanying figure (fig. 423.) The foetus (a), enclosed in its amniotic membrane (6), has its limbs as yet but very imperfectly formed, exhibiting pretty nearly the condition of a nascent Marsupial (vide fig. 418); but here it will be seen that the umbilical systems exhibit very striking differences in the two races. The vitellicle (f), with its pedicle (e), are of very small dimensions; the allantoid sac (g), on the contrary, is of considerable bulk, and, having ceased to act as a respiratory organ, becomes adapted to receive the urinary secretion through the canal of the urachus. The most important feature, however, is the rapid extension of the umbilical vessels (d), which in Birds and Marsupials were distributed only to the allantois, but in the placental Mammals these vessels rapidly spread over the chorion (h), and, coming in contact with the vascular surface of the womb, they soon form a new bond of communication between the mother and the foetus, constituting the placenta; and thus the offspring is nourished until, its intra-uterine growth being accomplished, it is born in an advanced condition of development, and becomes the object of maternal care during that period in which it is dependent upon the breast of its mother for support.