ONTOGENY 



241 



membrane. The instant a sperm strikes the vitelline membrane, 

 a jellylike substance begins to flow out from the cortical region 

 of the egg and continues to flow until the egg is surrounded by a 

 thick gelatinous envelope, except at the point where the sperm is 

 attached to the egg. At this point there is a funnel-shaped 

 depression in the jelly (Fig. 148). The jelly must not be con- 

 fused with fertilizin, which is soluble in sea water and is given 

 off from the egg independently. As the single sperm adheres to 



Fig. 148. — The egg of Nereis 12 minutes after insemination; the secretion 

 of the jelly is completed, the walls of the emptied alveoli of the cortical layer 

 appearing as radiating lines across the perivitelline space. The spermatozoon 

 lies in the funnel-shaped depression in the jelly; the egg having formed a fertili- 

 zation cone crossing the perivitelline space and touching the membrane beneath 

 the spermatozoon, re, fertilization cone; J, jelly; M, egg membrane; s, sperma- 

 tozoon. (After Lillie.) 



the egg by means of the perforatorium, the unsuccessful or 

 supernumerary spermatozoa are pushed away from the vitelline 

 membrane by the outflow of jelly. The jelly comes from alveoli 

 in the cortical zone of the egg directly beneath the vitelline 

 membrane and after the discharge of the jelly, a -perivitelline 

 space appears, traversed by delicate radiating strands. The 

 impact of the sperm with the egg produces a small puncture 

 in the vitelline membrane and evokes the formation of the 

 fertilization cone. The fertilization cone arises in the cytoplasm 

 opposite the point of attachment of the sperm and pushes out 



