THE PHYSIOLOGY OF FERTH^IZA TION 159 



Boveri (1895) has also observed that, if freshly ferti- 

 lized sea urchin eggs be broken into fragments by shak- 

 ing, those fragments which contain the egg nucleus 

 alone do not usually segment, though the nucleus en- 

 larges, dissolves, and reappears, but some such pieces 

 may segment once or twice and then stop (1902). 



It is evident, therefore, that the sperm sets in opera- 

 tion a progressive series of processes within the egg 

 and that complete activation is not attained by any 

 means at once after entrance of the spermatozoon, and 

 probably not until about the time of union of egg and 

 sperm nuclei. 



The question arises whether this is a purely quanti- 

 tative relation or whether the spermatozoon is involved 

 in a series of quahtatively different processes each of 

 which requires its aid or other extraneous support. 

 There is at present no evidence for the latter concep- 

 tion. On the quantitative side we would have the two 

 questions, whether the sperm activates by means of a 

 substance which it slowly releases, or whether it acti- 

 vates a substance, or ferment-like bodies, contained 

 within the egg. As Loeb has pointed out, if the first 

 assumption is correct we would expect that two sper- 

 matozoa would cause a more rapid progress of events 

 within the egg than a single spermatozoon; but this is 

 not the case; dispermic eggs segment in the same 

 tempo as monospermic eggs. 



The writer (191 2) has observed that portions of 

 sperm nuclei obtained by centrifuging Nereis eggs dur- 

 ing fertilization before penetration of the spermatozoon 

 produce an effect in terms of the sperm aster roughly 

 proportional to their size; this can be understood in 



