REPRODUCTION. 529 



of the trachea there is a complex larynx, the organ of 

 voice. 



Excretory System. — The kidneys are generally compact 

 and rounded bodies; the ureters open into the bladder, 

 except in Monotremes in which they enter a urinogenital 

 sinus. Except in Monotremes, the outlet or urethra of the 

 bladder unites in the male with the genital duct, to form a 

 urinogenital canal; while in the female, except in Mono- 

 tremes and a few other cases, the urethra and the genital 

 duct open into a common vestibule. 



Reproductive System. — In the more primitive mammals 

 the testes lie in the abdomen ; in the majority they descend 

 permanently (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 ; (fi) a 

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

 vaginal portion ending in the urinogenital aperture. In 

 Monotremes the two ducts are simple, and open separately 

 into the cloaca ; in Marsupials there are two uteri and two 

 vaginae; in Placental Mammals the uterine regions are more 

 or less united, and the vaginal regions are always fused. 



Development. — 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 ovum. In Monotremes the segmentation is 

 necessarily meroblastic, in other cases it is holoblastic. As 

 in Sauropsida there are two fcetal membranes — the amnion 

 and the allantois, both of which share in forming the pla- 

 centa of the Placental Mammals. 



Parturition. — The Monotremes are oviparous ; the Mar- 

 supials 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- 



