XX 



THE MECHANISM OF THE FORMATION OF THE 

 FERTILIZATION MEMBRANE' 



1. We can safely state that the previous experiments have 

 all clearly demonstrated one fact : that the initiation of develop- 

 ment in the sea-urchin egg is due to a change in the surface of 

 the egg — apparently a cytolysis of the cortical layer — ^which 

 results generally in a membraufe formation. In some cases 

 this membrane is more typically developed than in others. We 

 shall now communicate some experiments concerning the nature 

 of this process. 



The reader remembers from chap, ii, that if the mem- 

 brane formation proceeds slowly in the egg of S. j)urpuratus, 

 it begins with the formation of minute blisters at the surface 

 of the egg. These blisters grow in size and their contents fuse 

 while the siirface film of all the blisters forms the outer fertiliza- 

 tion membrane. This membrane is separated by the fluid (the 

 fused contents of the individual blisters) from the protoplasm 

 of the egg. What is the origin of this fluid ? Is it secreted by 

 the egg, or is it absorbed from the sea-water? If it were 

 secreted entirely from the egg, the diameter of the cytoplasm 

 should show a decrease after membrane formation. However, 

 measurements that I have made showed that the egg cytoplasm 

 undergoes no remarkable diminution of its volume at membrane 

 formation. Hence the essential part of the fluid that lies between 

 the cytoplasm and the membrane must be derived from without, 

 i.e., from the sea-water. But one part is derived from the egg, 

 and this latter part is, as we shall see, a colloid. 



1 Loeb, "Ueber die osmotlschen Eigenschaften und die Entstehung der 

 Befruchtungsmembran beim Seeigelei," Archiv f. Entwichlungsmechanik, XXVI, 

 S2, 1908. 



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