CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 327 



effects of the entrance of a spermatozoon into an egg is the pre- 

 vention of other spermatozoa from entering. If the spermato- 

 zoon enters the egg before the first polar body is cut off that polar 

 body as well as other cells which are formed from the egg are 

 rendered 'immune' to other spermatozoa. 



But although the influence of the entering spermatozoon 

 spreads so rapidly over the egg that within a few minutes at 

 most it renders all portions of the egg surface ' immune' to other 

 spermatozoa and thus prevents the fertilization of polar bodies 

 which are formed after fertilization, this influence does not go so 

 far as to cause the polar bodies to develop, even though such 

 polar bodies may be formed several hours after the spermatozoon 

 enters the egg. In Crepidula the second polar body is formed 

 about three hours after the entrance of the spermatozoon, and 

 during this time the sperm head has grown into a vesicular nu- 

 cleus and the sperm aster has become quite large, but in spite of 

 this the spermatozoon has not sufficiently affected the egg sub- 

 stance to cause the second polar body to develop even though 

 that body may contain the larger part of the egg protoplasm. 

 Only that portion of the egg develops which contains the sperm 

 nucleus and aster. 



This conclusion is similar in many respects to that reached by 

 Ziegler ('98), who found that when eggs of the sea urchin, Echinus 

 microtuberculatus, were constricted by cotton fibers under pres- 

 sure only that portion of the egg which contained the spermato- 

 zoon segmented while the portion containing the egg nucleus 

 never divided, though its nucleus frequently went through the 

 division phases, but without any division resulting. In this 

 case the portion of the egg containing the sperm might remain for 

 some time connected with the other portion by a narrow neck, 

 and yet the influence of the sperm in the one half did not cause 

 the other half to develop. 



These facts are of interest because of their bearing on the na- 

 ture of one of the processes concerned in fertilization. In a 

 series of important and extensive works on artificial partheno- 

 genesis and fertilization, which he has summarized in a recent 

 book, Loeb ('13) has shown that at least two factors are involved 



