XX 
THE MECHANISM OF THE FORMATION OF THE 
FERTILIZATION MEMBRANE! 
1. We ean 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 membrane 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. purpuratus, 
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 surface 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. 
1Loeb, ‘‘Ueber die osmotischen Eigenschaften und die Entstehung der 
Befruchtungsmembran beim Seeigelei,’’ Archiv f. Entwicklungsmechanik, XXVI, 
82, 1908. 
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