80 



FERTILIZATION OF THE FROG S EGG 



sperm nucleus is so indistinct that it cannot be located in sectioned 

 eggs. 



The middle piece of the spermatozoon, containing the centrosome, 

 enters the egg behind and attached to the sperm head or nucleus. As 



Point of sperm invasion 

 Pigmented cortex of 

 animal hemisphere 

 Penetration path 



Copulation path 



Yolk 



Pigmented cortex of 

 animal hemisphere 



Jelly 

 Penetration path 



Point of sperm entrance 

 Copulation path 



Invasion of the frog's egg by a spermatozoon. ( Top) The first step in fertiUzation, 

 the second {bottom) being syngamy or the fusion of the sperm and egg nuclei. 



this sperin head (nucleus) swells and loses its identity and establishes 

 the penetration path, the associated centrosome divides. The resulting 

 centrosomes establish an axis at right angles to the direction of sperm 

 progression. Frequently, however, the sperm penetration path will not 



