THE FERTILIZATION-PROCESS 



theless it can be discerned after this union. Finally, the 

 sperm-nucleus loses its visible identity through complete 

 fusion with the egg-nucleus. Soon thereafter two asters 

 arise, presumably from the single sperm-aster. 



The foregoing accounts of fertilization, embracing eggs 

 of the four classes made with respect to the period in matu- 

 ration when eggs are in the stage for reception of sperma- 

 tozoa, reveal two phenomena as common to all animal eggs. 

 First, after attachment of the spermatozoon to the egg- 

 surface, the egg-surface undergoes a change with the result 



/ 



C^ 



/W 



y; 



i^^ 



Fi 



G. 30. 



-Surface changes in the egg of Mitrocoma attending sperm-entry (after 



MetschnikofF). 



that the vitelline membrane becomes separated. The sur- 

 face-changes differ in quality : in eggs of Nereis a superficially 

 located jelly is extruded; in the Chaetopterus-egg the obser- 

 vable changes are less striking; in the egg of Amphioxus 

 discrete bodies flow together and liquefy before the mem- 

 brane separates widely from the egg] in sea-urchins' eggs 

 the briefly enduring surface-changes are most violent. In 

 order further to elucidate such surface-changes, I include 

 pictures of a Medusa-egg (Fig. 30) described by Met- 

 schnikofT.^ It should also be mentioned that the entrance- 



^ Metschnikoif, 1S86. 



177 



