THE FERTILIZATION-REACTION 



entry has supported the conclusion reached by Fol in his 

 study of sperm-entry into the eggs of the starfish and of 

 the sea-urchin, that the separation of the vitelline mem- 

 brane prevents the entrance of supernumerary spermatozoa. 

 The physical and chemical changes which take place in the 

 membrane after separation from the egg-surface are cer- 

 tainly such as would bar sperm-entry. But if the block 

 were purely mechanical, removal of the vitelline membrane 

 should make possible the entrance of extra spermatozoa 

 into the egg. It has been shown for the eggs of other ani- 

 mals, in addition to those of echinoderms, that disruption 

 or removal from fertilized eggs of the vitelline membrane 

 does not render the eggs any more liable to poly-sperm- 

 entry. ^ With membrane-separation the eggs undergo some 

 change and it is this change — not Its result, membrane- 

 separation — which constitutes the block to the entrance of 

 additional spermatozoa. Thus this block, which is more 

 subtle than the mechanical obstacle interposed by the 

 presence of a separated membrane, is established before 

 membrane-separation occurs. 



A rather exact indication as to the moment when the 

 block intervenes is furnished by my observations on the 

 &^g of Echinarachnius . Here one can follow the wave in 

 the ectoplasm which begins at the point of sperm-entry 

 and sweeps over the egg. As this wave progresses, it 

 renders the egg immune to the entry of supernumerary 

 attached spermatozoa. The behavior of these latter sper- 

 matozoa can be observed to change with the entrance of 

 the "fertilizing" spermatozoon into the egg: as the wave 



^ By -putting inseminated eggs in the best fertilizable condition 

 through bolting-silk at the 7noment zvhen they separate their mem- 

 branes one can most easily and with the least harm deprive them of 

 their membranes. Spermatozoa may be added inunediately or at 

 any time thereafter but each egg remains fertilized only by the single 

 spermatozoon of the first insemination. 



igg 



