STUDIES OF FERTILIZATION IN NEREIS 
I. THE CORTICAL CHANGES IN THE EGG: II. PARTIAL FERTILIZATION 
FRANK R. LILLIE 
From the Hull Zoological Laboratory, University of Chicago 
TEN FIGURES 
ONE PLATE 
I. THE CORTICAL CHANGES IN THE EGG 
In many animals one of the immediate effects of fertiliza- 
tion is to cause the egg to throw off a membrane, which is there- 
fore known as the fertilization membrane. This is the case for 
instance in the eggs of echinids and nematodes. In other cases, 
where a definite vitelline membrane exists prior to fertilization, 
cortical changes occur in the egg immediately after insemination, 
leading to the formation of a space, the so-called perivitelline 
space, between the protoplasm of the egg and the vitelline mem- 
brane. ‘This is the case for instance in the eggs of at least many 
annelids, molluscs and vertebrates, and it is unquestionably 
a more common phenomenon than the formation of a fertiliza- 
tion membrane. There can be little doubt that these apparently 
different phenomena are simply varying expressions of a change 
in the cortex of the egg, which is of the same nature in all cases. 
Loeb’s studies (09) have thrown much light on the nature of 
these cortical changes. In the case of the egg of Nereis they are 
relatively obvious in their character and readily followed. 
The ovocyte of Nereis is somewhat flattened in a polar direc- 
tion, measuring about 87.5 x 100u; it is girdled by a double 
equatorial zone of large oil drops. The large germinal vesicle is 
central and somewhat elongated in a polar direction. 
In his description of the unfertilized egg, Wilson (’92) distin- 
guished two membranes: a delicate outer vitelline membrane, 
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