CELL-DIVISION 



egg exhibits the strongest ectoplasmic changes ever shown 

 in its development. The periods of susceptibility thus 

 can be related to structural and behavior-changes in the 

 ectoplasm. No other constant change being associated 

 with the period of susceptibility than the visible and easily 

 demonstrable ectoplasmic state, we can assume that 

 ectoplasmic state is wholly or in part responsible for the 

 temporary lowered resistance of the egg to dilute sea-water. 



Not only did I fix the period of maximum susceptibility 

 of sea-urchins' eggs during their cleavage-cycle to hypo- 

 tonic sea-water; I was able also to localize precisely the 

 point on the eggs at which they break down during this 

 period. Properly to appreciate this latter finding, we need 

 fully to understand the structure of the hyaline plasma- 

 layer. This, part of the ectoplasm, though often the subject 

 of discourse by many writers, has never been properly 

 understood. I give therefore a detailed description of the 

 origin and structure of the hyaline plasma-layer in sea- 

 urchins' eggs, thus supplementing the description given in 

 the chapter on the ectoplasm. 



Selenka^ years ago described the surface of various echi- 

 noderm eggs as composed of a sheath of clear cytoplasm. 

 Later Hammar- and others also described this external 

 layer on fertilized sea-urchins' eggs. It is variously known 

 as the ectoplasmic layer, hyaline plasma-layer, Hammar's 

 layer, etc. It should not be, as it often is, confused with 

 the vitelline membrane.^ 



I have found that in several species of sea-urchins this 

 hyaline plasma-layer arises in the same way and has much 

 the same structure as described for the fertilized egg of 

 Arbacia. Into the periA^itelline space which arises with sep- 

 aration of the vitelline membrane fine filaments of the cyto- 



1 Selenka, 1S78, 1883. 

 - Hammar, i8g6, i8g/. 

 ' Just, iQjog, igjjc. 



277 



