826 MAMMALIA. 



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 such a large store of food being now superseded by the provision of 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 com- 

 mencement to their termination in the wrethro-sexual canal (h) a kind 

 of cloaca into which both the vaginae and the urethra empty themselves. 



(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 de- 

 velopment of the foetus is ultimately completed. 



(2440.) The ovary of a Marsupial animal, as has been already ob- 

 served, 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 impregna- 

 tion 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 Mam- 

 malia 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 sum- 

 mary 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 



* " On the Generation of Marsupial Animals, with a Description of the Impreg- 

 nated Uterus of the Kangaroo," by Kichard Owen, Esq., Phil. Trans. 1834. 



