228 PROBLEMS OF FERTILIZATION 



needed for activation; the spermatozoon is an inciting 

 cause of those reactions within the egg system on which 

 development depends. 



It is probable that the spermatozoon starts the acti- 

 vation of the egg before entering, and that penetration 

 of the spermatozoon into the egg is thereby facilitated; 

 penetration of the spermatozoon is not, as such, the 

 cause of activation. In the case of Nereis this is very 

 clearly seen because the spermatozoon remains external 

 for a long time after the egg has given numerous other 

 evidences of activation (see pp. 51-52). Loeb has also 

 shown that in the hybrid fertilization of sea urchin 

 eggs by starfish sperm the egg may in some instances 

 exhibit activation by membrane formation without 

 entrance of the sperm, which is then permanently ex- 

 cluded, as though the time for such form of reaction 

 on the part of the egg had passed. But in most animals 

 the act of inclusion of the spermatozoon within the egg 

 is very rapid, and membrane formation or other cortical 

 changes follow immediately. 



The first step in fertilization is a more or less specific 

 binding or agglutination of the spermatozoon to the 

 egg. We have seen that ova of sea urchins and of 

 some other forms secrete a substance that produces an 

 adhesive quality in spermatozoa and causes them to 

 agglutinate (pp. 112 fL). In 1913 I pointed out that "the 

 adhesive property that the sperm develops under these 

 circumstances may be an important factor in binding 

 the sperm to the egg until it can be incorporated," and 

 that this reaction furnished " evidence of an intimate 

 chemical combination of sperm and egg constituents 

 which begins at the very moment of union." 



