GENERAL LIFE OF MAMMALS. 637 



The ovaries are small. Except in Monotre/nes, 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 yoztng develop; and (c) a vaginal 

 portion ending in the urinoge?tital aperture. In Monotremes 

 the two ducts are si?nple, and open separately into the cloaca ; 

 in Marsupials there are two uteri and two vagina ; in 

 Placental Mammals the utei-ine regions are more or less 

 united, and the vaginal regions are always fused. 



I?i 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 is necessarily 

 meroblastic, iii other cases it is holoblastic. As in Sai/ropsida 

 there are two fxtal membranes — the amnion and the allantois, 

 both of which share in forming the placenta of the Placental 

 Mammals. 



The Monotremes are oviparous ; the Marsupials bring 

 forth their young prematurely after a short gestation ; the 

 Placental Mammals have a longer gestation, during which 

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

 means of the placenta. 



General Life of Mammals. 



Most Mammals live on dry land. The bats, however, 

 have the power of flight, and not a few forms, belonging to 

 diverse orders, are able to take long swooping leaps from 

 tree to tree. Thus, there are " flying phalangers," such as 

 Petaurus, among Marsupials ; " flying squirrels," such as 

 Pteromys, among Rodents; "flying lemurs" {Galeopithecus), 

 allied to Insectivores. Not a few are aquatic, — all the 

 Cetaceans, the two Sirenians, and the Pinniped Carnivores, 

 such as seals and walruses ; while water voles, beavers, 

 otters, polar bear, and many others are also at home in 

 the water. Burrowers are well represented by moles and 

 rabbits ; aboreal forms by squirrels and monkeys. 



As to the diet, man, most monkeys, pigs, and many others, 

 may be called omnivorous ; kangaroos, hoofed animals, and 

 most rodents are herbivorous ; the Echidna, the ant eaters, 

 hedgehogs and shrews, and most bats are insectivorous ; 



