AN EXPERIMENTAL ANALYSIS OF FERTILIZATION 

 IN PLATYNEREIS MEGALOPS. 



E. E. JUST. 



Study of the breeding habits of Platynereis megalops revealed 

 the fact, as has been pointed out (Just, '14), that insemination 

 takes place in the body cavity of the female and that although 

 egg laying begins often but five seconds after copulation, the eggs 

 will not fertilize when artificially inseminated after exposure to 

 the action of sea-water. It is this failure of sea-water insemina- 

 tion that forms the basis of the present contribution to the 

 analysis of fertilization in Platynereis. In order clearly to inter- 

 pret the phenomena of sea-water insemination a study of the 

 morphology of the normal fertilization was made (see Just, '15a). 



The experiments undertaken for the analysis of fertilization 

 in Platynereis come under three heads: 



A. Conditions of successful insemination. 



B. Cross fertilization with Nereis. 



C. Artificial parthenogenesis with various agents. 



B and C are taken up mainly because they supplement results 

 under A. 



A. Conditions of Successful Insemination. 



During the summer of 191 1, I was studying the maturation and 

 fertilization of the Platynereis egg for comparison with those 

 processes in Nereis. The methods of insemination used with 

 Nereis, cutting out the eggs and sperm in sea-water, gave no 

 cleavage. Various trials with the utmost care, using diverse 

 methods never gave cleavage. Not until August 24, 191 1, did 

 I chance to find that normally insemination takes place in the 

 body cavity of the female (cf. Just, '14). 



I . Observations on Eggs Inseminated in Sea-water. 



If eggs and sperm be cut out of Platynereis and mixed in sea- 

 water, the phenomena of maturation, sperm attachrrient, and 



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