16 HEREDITY AND ENVIRONMENT 



usually broken off and left outside. The 

 nucleus in the head of the sperm then begins 

 to absorb material from the egg and to grow 

 in size and at the same time a minute granule, 

 the centrosome, appears, either from the mid- 

 dle-piece or from the head of the sperm, and 

 radiating lines rim. out from the centrosome 

 into the substance of the egg. The sperm 

 nucleus and centrosome then approach the egg 

 nucleus and ultimately the two nuclei come to 

 lie side by side (Figs. 4-7) . Usually when one 

 spermatozoon has entered an egg all others are 

 barred from entering, probably by some 

 change in the chemical substances given out by 

 the egg. 



This union of a single spermatozoon with an 

 egg is known as fertilization. Whereas egg 

 cells are usually, but not invariably, incapable 

 of development without fertilization, there be- 

 gins, immediately after fertilization, a long 

 series of transformations and differentiations 

 of the fertilized egg which leads to the develop- 

 ment of a complex animal — of a person. In 

 the fusion of the egg and sperm a new indi- 

 vidual, the oosperm, comes into being. The 



