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 
Jew 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 utert and two vagine ; in 
Eutherian Mammals the uterine regions ave 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, within a swelling or 
Graajian follicle, which eventually bursts and liberates the 
egg-cell. In Monotremes the segmentation, as might be 
expected, 1s meroblastic » in other cases tt is holoblastic. As in 
Sauropsida, there are two fetal membranes—the amnion and 
the allantots, both of which share in forming the placenta of 
the Placental Mammals. In Marsupials the allantois ts 
usually small and degenerate. 
The Monotremes are ovtparous; the Marsupials bring 
jorth their young prematurely after a short gestation, but a 
true allantotc 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. y 
Ln 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 MamMMaLs 
The rabbit (Lepus cuniculus) is a familiar representative of 
the Rodent order, to which rats and mice, voles and beavers, 
