74 



FERTILIZATION OF THE FROG S EGG 



Amplexus in the toad: Bujo jowlcri. Pectoral grip. 



eter. Polyspermy, or multiple sperm invasion of the egg, occurs 

 naturally in some telolecithal eggs such as those of the bird. It is 

 also common in urodele eggs, but not so with the Anura (e.g., the 

 frog). More effective than the jelly in limiting fertilization to one 

 sperm is the negative chemical and/or physical reaction of the egg 

 toward any extra spermatozoa after one of them has made contact 

 with the egg surface. Simultaneously with sperm activation there ap- 

 pears an immune reaction of the egg to the invasion of any accessory 

 spermatozoon, even of the same species. In any case, one and only 

 one spermatozoon nucleus normally fuses with the egg nucleus. Any 

 accessory spermatozoa which are successful in invading the egg 

 attempt to divide independently of the egg nucleus and then degen- 

 erate. Extensive polyspermy, which may occur in aged eggs, inter- 

 feres drastically with the cleavage mechanism and the eggs reach an 

 early cytolysis and death. 



The effective frog spermatozoon always enters the egg in the 

 animal hemisphere. This does not deny the possibility of sperm en- 

 trance in the metabolically more sluggish vegetal hemisphere. The 

 sperm nucleus which conjugates with the egg nucleus is one that 



