THE RABBIT AS A TYPE OF MAMMALS. 697 



except in Monotremes and a few other cases, the urethra and 

 the genital duct open into a common vestibule. 



In the more primitive Mammals the testes lie in the 

 abdomen ; in the majority they descend permanently (or in a 

 few cases temporarily) into a single or paired scrotal sac, 

 lying, except in Marsupials, behind the penis. 



The ovaries are small. Except in Monotremes, the genital 

 ducts of the female are differentiated into (a) Fallopian tubes, 

 which catch the ova as they burst from the ovaries ; (b) a 

 uterine portion in which the young develop ; and (c) a vaginal 

 portion ending in the urogenital aperture. In Monotremes 

 the two ducts are simple, and open separately into the cloaca ; 

 in Marsupials there are two uteri and two vagina; in 

 Eutherian Mammals the uterine regions are more or less 

 umted, and the vaginal regions are always completely fused. 



In Monotremes the eggs are large and rich in yolk ; in all 

 others they are small and almost yolkless. In the ovary each 

 ovum lies embedded in a nest of cells, within a swelling or 

 Graafian follicle, which eventually bursts and liberates the 

 egg-cell. In Monotremes the segmentation, as might be 

 expected, is meroblastic ; in other cases it is holoblastic. As in 

 Sauropsida, there are two fatal membranes the amnion and 

 the allantois, both of which share in forming the placenta of 

 the Placental Mammals. In Marsupials the allantois is 

 usually small and degenerate. 



The Monotremes are oviparous; the Marsupials bring 

 forth their young prematurely after a short gestation, but a 

 true allantoic placenta may be represented, as in Perameles \ 

 the Eutherian Mammals have a longer gestation, during 

 which the young are vitally connected to the wall of the uterus 

 by means of the placenta, which is always well developed, and 

 of great importance in the nutrition of the embryo. 



In all Mammals the young are for a longer or shorter 

 period dependent upon the milk secreted by the mammary 

 glands of the mother ; in Marsupials this dependence is 

 especially marked. 



THE RABBIT AS A TYPE OF MAMMALS 



The rabbit (Lepus cuniculus) is a familiar representative of 

 the Rodent order, to which rats and mice, voles and beavers, 



