312 HUMAN PHYSIOLOGY 



3. FERTILIZATION 



The spermatozoa discharged during the act of coition into 

 the vagina of the female pass through the uterus and 

 oviduct into the upper part of the oviduct, called the am- 

 pullae. This is an active movement and takes place in the 

 direction opposite to that of the movement of the cilia of the 

 epithelium. 



After discharge of the egg fertilization takes place, gen- 

 erally in the ampulla, by the entrance of one of the sperma- 

 tozoa into the egg. The spermatozoon forces its way 

 through the membrane of the egg and proceeds in a radial 

 direction towards its centre. The tail of the spermatozoon 

 becomes dissolved in the egg, while the head becomes the 

 male pronucleus. The male and female pronuclei increase 

 in size and approach each other. They are now similar in 

 appearance, so that they can no longer be distinguished. 



After losing the nuclear membrane the nuclear fibres of 

 each nucleus break up into a number of loop-shaped pieces, 

 and these fragments are mixed. From the thus united egg 

 and spermatozoon the new individual develops by nucleus- 

 and cell-division and cell differentiation. 



While the unfertilized eggs are soon destroyed, the fertil- 

 ized egg, passing in about three days through the duct 'into 

 the uterus, is held in the uterus. It sinks between the folds 

 of the mucous membrane of the uterus, which is greatly 

 thickened. The walls of the fold unite with the membrane 

 of the egg and cover the egg. This part of the mucous 

 membrane subsequently forms the placenta. 



