1 6 Vertebrate Embryology 



that the processes of maturation and fertiliza- 

 tion offer a wide field for speculation, but one 

 into which it is not within the province of this 

 book to enter. 



Fertilization of the Egg 



As the eggs are extruded from the cloaca of 

 the female, which process may take place in a 

 few minutes or may be prolonged over several 

 days, the spermatozoa are spread over them 

 by the male and at once begin to bore their 

 way through the jelly towards the eggs. 



The exact nature of the changes that take 

 place after the sperm enters the egg has not 

 been entirely determined, but the essential 

 points will be given. A few minutes after 

 coming in contact with the vitelline membrane, 

 the head of the spermatozoon works its way 

 into the egg and moves towards the female 

 pronucleus, with which it fuses to form the 

 so-called segmentation nucleus. The head of 

 the spermatozoon, after it has entered the 

 egg, is known as the male pronucleus, and the 

 essential act of fertilization is the fusion of 

 the male and female pronuclei. The tail of 

 the spermatozoon remains outside of the egg 

 and apparently takes no part in the process 



