THE BIOLOGY OF THE CELL SURFACE 



but properly the vitelline membrane. I have repeatedly 

 followed the process in the egg of Arbacia, though it is not 

 the most favorable form for this study. In the eggs of the 

 genus Strongylocentrottis, of Echinus and of Echinarachnius, 

 for example, the separation of the membrane is more easily 

 followed. 



If eggs of Echinarachnius be inseminated with thin sperm- 

 suspension, they throw off their membranes that are 



d e 



Fig. 1 8. — Successive stages of membrane-separation in the living egg of Echi- 

 narachnius parma, site of sperm-entry. 



fully lifted and equidistant from the egg-surface in about 

 two minutes. Membrane-separation follows sperm-pene- 

 tration. After the sperm-head has entered the ectoplasm, 

 a blister appears on the egg-surface at the point of sperm- 

 entry. This blister contains drops that move toward the 

 membrane and go into solution. The process is gradually 

 continued throughout the whole ectoplasm, and thus the 

 membrane is lifted in a wave that sweeps over the surface 

 (Fig. 1 8). By break-down of material in the ectoplasm, 

 beginning at the site of sperm-entry, the membrane is 

 separated from the surface. After this membrane is fully 



io6 



