518 MAMMALIA. 



autumn and copulation then takes place, but it is not until spring 

 that ovulation occurs, so that the spermatozoa live all the winter 

 in the uterus. It is recorded, however, that young female bats 

 experience their sexual season in the spring (compare the case 

 of Salamandra, p. 307). 



The occurrence and frequency of the sexual season also vary 

 to a considerable extent in different mammals. In many it 

 occurs more than once in each year, in others not more than once 

 in two years. Climatic and nutritive conditions are important 

 factors in determining its onset and periodicity, and there is 

 considerable variation in accordance with local conditions even 

 amongst individuals of the same species. Thus woodland deer 

 breed earlier than those which live above the tree-level, and 

 foxes at a high altitude breed later than those which live in the 

 plains. Walruses, which are compelled to expend their energies 

 for twelve months in feeding their calf, breed only once in two 

 years. Scarcity of food and a rigorous climate tend to reduce 

 sexual activity, while the luxury attending domestication re- 

 sults in greatly increased breeding powers. 



The graafian follicle after expelling its ovnm becomes filled with cells 

 and is called the corpus luteum. This structure undergoes a series of 

 changes which difJer (Homo) according to the occurrence or non-occur- 

 rence of gestation. Considerable importance has been ascribed to these 

 changes (ovarian secretion *), and it is said that destruction of the corpus 

 luteum by electric cautery or other means will bring about cessation of 

 development and degeneration of the uterine foetus. 



The ova of the Mammalia were first discovered by C. E. von 

 Baer. They are extraordinaril}^ small (with very little yolk) and 

 are surrounded by a strongly refractile membrane (zona radiata), 

 round which a layer of albumen is sometimes deposited in the 

 oviduct. 



The fertilization appears to take place in the oviduct (fallo- 

 pian tube), and segmentation, which except in Monotremata is 

 complete, is effected either in the oviduct (rabbit, mole, etc.) or 

 in the uterus (ferret, sheep, etc.). Amnion and allantois are pre- 

 sent. In the uterus the embryo becomes connected with the 

 uterine wall by means of its outer epithelial layer, now known as 

 the trophoblast. This, later, becomes coated wholly or in part 

 on its inner side by somatic mesoblast and constitutes the mem- 

 brane known as the subzonal membrane (false amnion and 



* Marshall, op. cit. 



