MAMMALIAN CHARACTERISTICS 783 



Eiitherian Mammals the uterine regions are more or less 

 united, 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 y 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 foetal membranes — the amnion 

 and the allantois, both of which share informing 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 7vell 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 -SLnd Lagomys are some- 

 times ranked as Duplicidentata, in contrast to all other 

 Rodents (Simplicidentata). To illustrate habit diiferences 

 between nearly related types, it is useful to contrast rabbit 

 and hare. The hare does not burrow ; its young are born 

 furred and open-eyed ; it runs in a different way ; its 

 danger-signal is made with its teeth, and so on. 



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

 herbivorous, and often leaves softer food for the succulent 



