STUDIES OF FERTILIZATION 365 
extent of the membrane; it is more like an electrical discharge 
or some other physical disturbance than a chemical effect. 
J. Loeb (’09) has formed the hypothesis that the cortical layer 
of the egg, especially of sea-urchins, is an emulsion which is 
rendered stable by a third substance consisting of lipoids, espe- 
cially cholesterin. ‘The emulsion becomes unstable on solution of 
the lipoids; this enables the albuminous drops, which he conceives 
to form one phase of the emulsion, to take up water; hence the 
layer liquefies and the perivitelline space arises; the fertilization 
membrane is thus formed. Hence, according to Loeb, the action 
of lipoid-dissolving substances is to cause the formation of the 
fertilization membrane. Without committing ourselves to these 
specific views of the nature of the cortical emulsion, which Loeb 
himself does not hold very strongly, we may admit that Loeb’s 
hypothesis, that the formation of the fertilization membrane is 
due to the breaking down of a cortical emulsion, fits the case of 
Nereis very well. If we go further, however, we must note an 
important lack of agreement with Loeb’s hypothesis. As Loeb 
himself points out, the theory implies that the membrane of the 
egg is permeable for sea-water and crystalloid substances, and on 
the other hand impermeable for colloids; in Nereis the contents 
of the cortical alveoli are unquestionably colloid, as Loeb’s 
hypothesis requires, but it is perfectly certain that they diffuse 
through the membrane to form the external jelly; at the same time, 
unquestionably, sea-water enters to take the space previously 
occupied by the colloid. The membrane is therefore permeable for 
both erystalloids and colloids at this time. I have not, however, 
investigated farther the properties of the egg membranes and 
must leave this problem to those who are better qualified as phys- 
iologists to make such a study. 
It would appear that the presence of this colloid substance 
in the cortex is an inhibition to the maturation of the egg, because 
as soon as it is removed, maturation processes are set in motion 
and both polar bodies are formed. In what manner it inhibits 
is of course problematical. In the egg of Ascaris megalocephala 
there is a similar excretion of a cortical colloid which forms, in 
this case, the thick resistant perivitelline membrane. The ap- 
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