STUDIES OF FERTILIZATION 567 



10. INTERPRETATION OF SOME PHENOMENA OF NORMAL 

 FERTILIZATION IN NEREIS 



To observe all the details of normal fertilization it is desirable 

 to inseminate in a suspension of India ink which will define the 

 transparent substances exuding from the egg on insemination, 

 the following observations can then readily be made on mixing 

 a drop of the eggs in the ink suspension with a drop of opalescent 

 sperm suspension : Hundreds of spermatozoa become attached to 

 each egg almost immediately; those in contact with the egg do 

 not show much activity, but are usually definitely oriented radi- 

 ally; the spermatozoa external to these are in active movement. 

 In about a minute a clear fluid begins to exude from the egg and 

 surrounds all attached spermatozoa and involves the immediate 

 neighbors. The first exudate is quite fluid for it flows around the 

 spermatozoa and does not sweep them away, but the movements 

 of all the spermatozoa within the exudate cease suddenly. The 

 flow continues, and then most of the spermatozoa are swept away 

 from contact with the egg, for the later exudate is gelatinous in 

 consistency. However, a good many spermatozoa remain in 

 contact with the egg for some time, but these are detached one by 

 one as the flow of the jelly continues, until only one remains. 

 Some of the supernumerary spermatozoa are not carried away 

 until five or more minutes after insemination. 



The immediate prevention of polyspermy in Nereis appears 

 to be due to the paralyzing effect of the egg-exudate poured out 

 in response to the stimulus of the first effective spermatozoon. 

 Polyspermy could take place in Nereis only under two conditions, 

 namely, (1) if two or more spermatozoa simultaneously give the 

 stimulus to the egg that causes excretion of the agglutinin; for 

 the condition of stimulus appears to be that the spermatozoon be 

 securely anchored to the egg; and all spermatozoa not securely 

 attached at the moment the egg begins to secrete are prevented 

 from securing attachment by the resulting paralysis; (2) if the 

 reaction of the egg be slow and therefore localized at first to the 

 region of an effective spermatozoon, opportunity will be afforded 

 for attachment of other spermatozoa. 



