THE FERTILIZATION-PROCESS 



no longer visible in the living egg; the spermatozoon, seen 

 with difficulty in the period during which these changes take 

 place, becomes again visible after they pass over. About 

 fifty minutes after insemination, the sperm-head disappears 

 within the egg, leaving the tail and middle-piece outside. 



Fig. 25. — Living egg of Nereis in a suspension of Chinese ink in sea-water, fifteen 

 minutes after insemination (after Lillie). 



About five minutes later follows the extrusion of the first 

 polar body. The second is extruded about fifteen minutes 

 later. The egg cleaves into two unequal blastomeres one 

 hour twenty-five minutes after insemination. 



Many details of the fertilization-process in the egg of 

 Nereis can be observed only by fixing the eggs with a suit- 



161 



