EGG NUCLEUS 



FERTILIZATION CONE 



SPERM 

 PARTLY ENGULFED 



Fig. 21. The process of fer- 

 tilization. Top: Many sperm 

 reach surface of egg; one 

 sperm starts wave of negativ- 

 ity which excludes all other 

 sperm. Center: The egg reacts 

 to the sperm by forming a 

 fertilization cone which en- 

 gulfs the sperm. Bottom: The 

 sperm head is inside the egg 

 and the egg has formed a fer- 

 tilization membrane within 

 which cleavage and early de- 

 velopment take place. 



TAIL OUTSIDE 



FERTILIZATION 

 MEMBRANE 



SPERM NUCLEUS 



EGG NUCLEUS 



46 



