714 MAMMALIA. 



(jig. 329). The ovaria (a, a) are now reduced to compara- 

 tively 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 yolks peculiar to ovipa- 

 rous 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 of food being now superseded by the provision of another kind of 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 (6), in which the first part of gestation is 

 accomplished. The two uteri open by two orifices (e, f) into 

 the two vaginae (g, g), which remain quite distinct from each other 

 from their commencement to their termination in the urethro- 

 sexual canal (A), a kind of cloaca into which both the vaginae and 

 the urethra empty themselves. 



(839.) Such being the arrangement of the generative apparatus 

 of the female Kangaroo, we are prepared, in the next place, to con- 

 sider 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 developement of the fetus is ultimately completed. 



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 yolk 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 Graqfian 

 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 yolk. The deve- 

 lopement 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 yolk is required 

 to contribute to the nourishment of the newly-formed being, in 

 the Mammifera where no adequate supply of yolk 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 or- 

 dinary Mammal a placenta is developed, forming a means of vascular 

 communication between the mother and the fetus. 



(840.) The important investigations of Professor Owen upon 



