THE FERTILIZATION-REACTION 



polyspermy does not occur. Eggs in the anterior segments 

 of the worm, where most spermatozoa are found, are as free 

 from polyspermy as those which first issue from the anal 

 segment. 



]\Iy preparations of starfish eggs which show more than 

 one spermatozoon entering are of eggs fertilized either before 

 break-down of the germinal vesicle or after second matura- 

 tion. Never in the stage of first maturation when the eggs 

 are in the stage of optimum fertilizability is there poly- 

 sperm-entry. If, however, fertilizable eggs be injured, 

 supernumerary spermatozoa enter. 



These observations lend themselves to the conclusion 

 that the block to polyspermy is rapidly established with the 

 attachment or entry of one spermatozoon. 



Now into all these eggs — of Echinarachnius, Arhacia^ 

 Platy7iereis, Asterias — the spermatozoon may enter at any 

 point. They thus differ from some other monospermic eggs, 

 ~as^ those of the trout or the ink-fish, which possess a tube, 

 the micropyle, through which the spermatozoon gains 

 entrance; or such eggs, as of Ciona and of Amphioxus, into 

 which spermatozoa enter always at the vegetal pole. 

 Theoretically, therefore, these eggs under discussion are 

 polyspermic; until they establish the block against super- 

 numerary spermatozoa any part of the eggs' surface is 

 responsive to sperm-attachment — leaving aside the unlikely 

 assumption that one site alone admits the spermatozoon 

 and that this would vary even in eggs of the same female. 

 From this point of view the interposition of the block to 

 polyspermy becomes highly significant for it oflFers a means 

 by which we can set the termination of the initial reaction 

 between normal egg and normal spermatozoon of any of 

 the species named: it ends with sperm-attachment.^ The 



^ This refers to normal eggs, it must be understood, since sperma- 

 tozoa may enter unfertilizable eggs — i.e., immature ones. See 

 Iwanzoff and others. 



201 



