50 



EMBRYOLOGY 



BLOCKING LAYER 



. ENZYME BLOCKED 



SPERM 

 + OR — ♦ 

 PARTHENOGENETIC AGENT 

 St 

 SUBSTRATE 



blocking laye.r dissolved 

 nzyme free^^ 



"b. 



SUBSTRATE 

 IN CONTACT 



GRANULES IN CORTEX 



GRANULES DISSOLVED 



PARTHENOGENETIC AGENTX ; 



Fig. 22. Top: Hypothetical diagram of the release of a substance during 

 fertilization. A is an enzyme which is covered by a surface layer, B. A substance 

 C, which is a normal substrate for enzyme A is prevented by B from coming 

 in contact with A. Assume that a substance from the sperm or a parthenogenetic 

 agent causes layer B to dissolve. Then A and C come together and a chemical 

 reaction takes place. Bottom: Actual disappearance of granules in the periphery 

 of the egg at fertilization and also after the action of some parthenogenetic 

 agents. 



materials are assumed to be present for a reaction to occur between A and C, 

 whatever this reaction may be, but the compounds cannot interact because 

 of a block at the surface of one of them. Anything that will remove 

 this block — that is, take off the surface layer, B — will allow A and C 

 to come together. A reaction can then take place, and development can 

 proceed. 



This kind of hypothesis makes it clear why a wide variety of agents can 

 stimulate an egg. There is a B substance, a thin layer, around A, and it is 

 easy to see that acids, bases, strong salts, and a variety of agents might 

 remove B or change it in some way so that it comes off the surface, freeing A 

 to react with C. 



