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.
(2435). We select the Kangaroo as an example of the entire group, beginning, as we have hitherto done, with the organization of the male organs of generation.
(2436). The first circumstance that strikes the attention of the anatomist in a male Marsupial is the extraordinary position of the testes, which, instead of being situated behind the penis, as in most placental Mammals, are placed in front of that organ in a kind of scrotum that occupies the same place as the pouch of the female, and is in like manner supported by two marsupial bones derived from the pubes, around which the cremaster muscle winds in such a manner as to enable it powerfully to compress the testicles during the congress of the sexes. The vasa deferentia derived from the testes open into the commencement of the urethra, which now, for the first time, forms a complete canal leading from the bladder to the extremity of the penis. The auxiliary glands that pour additional secretions into the urethra are of great size, and more numerous than those met with in the human subject. In the first place, the commencement of the urethral tube is embraced by a bulky and conical prostate, to which succeed three pairs of large secreting organs (Cowper's glands), each enveloped in a musculo-membranous sheath, apparently intended to compress their substance, and thus efficiently discharge their secretion into the canal of the urethra, there to be mixed up with the seminal fluid.
(2437). But perhaps the most decided peculiarities that characterize the males of Marsupial quadrupeds are met with in the construction of the penis itself. The two roots or crura of the corpora cavernosa are not, as in the higher Mammals, attached to the branches of the ischium by ligamentous bands, but each swells into a large bulb enclosed in a powerful muscular envelope. The bulbous portion of the urethra is likewise double, and embraced by powerful muscles. In the Kangaroo, moreover, the spongy erectile tissue that encloses the urethra passes with that canal through the centre of the body of the penis, formed by the corpora cavernosa, so that a glans can scarcely be said to exist; but in other Marsupials, as, for example, in the Opossums (Didelphys), the extremity of the intromittent organ is bifid, thus forming another approximation to the oviparous type.
(2438). In the female Kangaroo, and other Marsupials, there are still two distinct uteri, opening into the vagina by distinct orifices; and even the vagina itself is double, exhibiting a very peculiar and interesting arrangement, represented in the subjoined figure (fig. 416.) The ovaria (a a) are now reduced to comparatively small dimensions when compared with those of the Ovipara - a circumstance that depends upon the reduced size of the ovarian ovules, which no longer present the bulky yelks peculiar to oviparous generation, the necessity for the existence of such a large store of food being now superseded by the provision of another kind of nourishment derived from the mammary glands. The Fallopian tubes commence by wide fimbriated apertures; and each leads into a separate uterine canal (b, c), in which the first part of gestation is accomplished. The two uteri open by two orifices (e, f) into the two vaginae (gg), which remain quite distinct from each other from their commencement to their termination in the ureihro-sexual canal (h) - a kind of cloaca into which both the vaginae and the urethra empty themselves.
Fig. 416. Generative organs of the female Kangaroo.
(2439). Such being the arrangement of the generative apparatus of the female Kangaroo, we are prepared, in the next place, to consider the structure of the Marsupial ovum, and to trace its progress from the ovary, where it is first formed, into the Marsupial pouch, where the development of the foetus is ultimately completed.
(2440). The ovary of a Marsupial animal, as has been already observed, resembles that of ordinary Mammalia, and presents the same dense structure. But the ovarian ovules, although characterized by the paucity of yelk as compared with the oviparous classes, yet have a larger proportion than exists in the placental Mammalia. When impregnation is effected in the Marsupial animal, the Graafian vesicle or ovisac is ruptured, and the little ovulum escapes into the Fallopian tube, whereby it passes into the uterine cavity, from whence, of course, it must absorb the materials destined to support the future embryo, in the same manner as the egg is furnished in the oviduct with the albumen that invests the yelk. The development of the embryo from the blastoderm or germinal membrane is, no doubt, accomplished in the same manner in all Mammalia as it is in Birds, up to a certain stage of maturity: but at that stage of growth when, in the case of the Bird, the yelk is required to contribute to the nourishment of the newly-formed being, in the Mam-mifera, where no adequate supply of yelk exists, other means must be resorted to; and accordingly the Marsupial embryo is born prematurely, in order to supply it with milk; and in the ordinary Mammal a placenta is developed, forming a means of vascular communication between the mother and the foetus.
(2441). The important investigations of Professor Owen upon this subject* cannot be too highly appreciated. In the gravid uterus of a Kangaroo, examined by this indefatigable labourer in the cause of science, a foetus was met with that had apparently arrived very nearly at the term of its intra-uterine existence; and the following is a summary of its anatomy at this period.
(2442). The ovum (fig. 416, c) was lodged in one of the uterine cavities, and the foetus was about an inch and four lines in length. The walls of the gravid uterus were obviously dilated, and its parietes varied in thickness from one to two lines, being in the unimpregnated state about half a line; but this increase was not in the muscular coat, but in the lining membrane, which was thrown into irregular folds and wrinkles. There was, however, not the slightest trace of any vascular connexion between the uterus and the ovum - neither placenta nor villi, nor any determination of vessels to a given point on either of the opposed surfaces of the chorion or uterus: on the contrary, the external membrane of the ovum (chorion) exhibited not the slightest trace of vascularity, even under the microscope, and seemed in every respect to resemble the membrana putaminis that lines the egg-shell.