672 MAMMALIA. 



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 

 united, and the vaginal regions are ahvays completely fused. 



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

 others they are small and almost yolk/ess. 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 ivhich 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 itterus 

 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, 

 lemmings and marmots, also belong. Like the hare (Lepus 

 timidus] and other species of the same genus, and like the 

 Picas or tailless hares (Lagomys), the rabbit has two pairs of 

 incisors in the upper jaw, while other Rodents have a single 

 pair. Therefore the genera Lepus and Lagomys are some- 

 times ranked as Duplicidentata, in contrast to all other 

 Rodents (Simplicidentata). 



With the rabbit's mode of life all are familiar. It is herb- 

 ivorous, and often leaves softer food for the succulent bark 

 of young trees ; it is gregarious and a burrower ; it is very 



