THE EARLY DEVELOPMENT OF THE MAMMAL 379 



internal ovarian secretions (hormones). Usually, too, the 

 period of oestrus is preceded by a prooestrus, or menstruation. 

 In man, however, these two conditions are more or less inde- 

 pendent, and no very constant relation is apparent. In the 

 rabbit, cat, and ferret, ovulation occurs only after coitus 

 (nine to ten hours after, in the rabbit, and usually within 

 fifty hours in the cat), while in the dog, rat, and mouse, and 

 in many Ungulates and Primates, it occurs independently of 

 coitus. In many cases ovulation follows upon parturition, in 

 the mouse about fourteen hours after the birth of a litter, and 

 in the rat within about eighteen hours. 



II. FERTILIZATION AND THE EARLIER PHASES OF 

 DEVELOPMENT 



1. Fertilization 



In most of the Mammals fertilization occurs in the upper part 

 of the oviduct, almost immediately upon the entrance of the 

 ovum. After the introduction of the spermatozoa into the 

 vagina, they make their way to the upper ends of the oviducts, 

 where they may remain alive and capable of functioning for 

 several days or even weeks. Apparently the ovum too may 

 remain in the oviduct, alive and capable of development, for 

 several days or even a fortnight, in case fertilization does not 

 occur at once. 



The details of the sperm entrance, the formation of the sec- 

 ond polar body, the establishment and fusion of the egg and 

 sperm pronuclei, and the formation of the first cleavage figure, 

 are not unusual and need not be described here (Fig. 146). 

 Monospermy is typical of the Mammalia, and since there is no 

 micropyle, the spermatozoon has to penetrate the zona pellucida 

 (radiata), as well as the corona radiata, which may remain 

 surrounding the egg during fertilization and even until a late 

 cleavage stage. Long before it finally breaks down it becomes 

 soft and easily penetrated (Fig. 146). In the rat the vigorous 

 movements of the spermatozoa within and among the cells 

 of the corona tear this to pieces and leave the egg naked after 



