302 MVRA MELISSA SAMPSON. 



Hopkins Marine Laboratory at Pacific Grove, California. I wish 

 to express my appreciation of the hospitality extended to me at 

 these laboratories, and my thanks to their Directors, Dr. Frank 

 R. Lillie and Dr. Walter K. Fisher, for their aid and encourage- 

 ment. 



The fact that the spermatozoa, rather than the medium which 

 carries them, are the essential agents in fertilization was estab- 

 lished in 1824 by Prevost and Dumas; and it is now conceded 

 that fertilization is monospermic and involves the combination 

 of nuclear material from a single spermatozoon and a single 

 ovum. In many species only a single spermatozoon penetrates 

 the cytoplasm of the egg, as was shown by Hertwig (1876); 

 but even when in normal fertilization more than one enters the 

 egg, as in the pigeon, the pro-nucleus of only one enters into 

 the formation of the fertilization nucleus (Harper, 1904, and 

 Blount, 1909). As stated by Glaser (1915), "a single sperma- 

 tozoon is sufficient to carry on the biparental effect." 



It is well know r n, however, that in the process of fertilization 

 in any species the spermatozoa far exceed the ova in number. 

 This was first noted in frogs by Spallanzani (1785), and verified 

 by Prevost and Dumas (1824). Under such conditions direct 

 contact of every ovum with a single spermatozoon is possible. 

 But mere contact does not ensure fertilization. Even in physio- 

 logical solutions spermatozoa rapidly lose their fertilizing power, 

 Vernon (1899); Gemmil (1900); Dungay (1913); Lillie (1915) 

 and Cohn (1918). Two explanations have been offered in recent 

 years to account for the excess of spermatozoa. Lillie (1915) 

 implies that in an excess at least one spermatozoon possessing 

 sufficient fertilizing power ("sperm receptors") will reach each 

 ovum. On the other hand Glaser (1915) and Cohn (1918) state 

 that the extra spermatozoa may produce changes in an ovum 

 which facilitate the inclusion of a single spermatozoon. 



The idea of a superficial effect of sperm on ova is supported by 

 results obtained with Nereis limbata, Lillie (1911) and Goodrich 

 (1920); with Arbacia punctulata, Lillie (1921); with Sabellaria 

 alveolata and Halosydna gelatinosa, Labbe (1921, 1922) ; and with 

 Asterias, Chambers (1923). There are indications that a 

 chemical substance or substances are produced by the sperm: 



