364 FRANK R. LILLIE 
remain as delicate strands of protoplasm uniting the vitelline 
membrane to the surface of the egg. The jelly, therefore, repre- 
sents the alveolar contents only of the cortical layer of the unfer- 
tilized egg, and the perivitelline space is nothing but the contracted 
alveoli of the cortical layer filled with fluid. The perivitelline 
space must, therefore, be regarded as intraovular with a delicate 
external cytoplasmic wall lining the vitelline membrane; this we 
may distinguish as the plasma membrane; it is comparable in 
some respects to the fertilization membrane of sea-urchins. 
Unfertilized eggs allowed to stand in the sea-water form no 
jelly and retain the cortical layer; the germinal vesicle remains 
intact; but, if they be strongly centrifuged, the jelly forms, the 
perivitelline space arises, the germinal vesicle breaks down and 
both polar bodies are formed; but parthenogenetic development, 
usually at least, does not take place. Similarly, the addition of 
a fairly strong solution of potassium chloride to the sea-water 
causes formation of the jelly and the perivitelline space while the 
eggs are still in the solution; maturation takes place after trans- 
fer to sea-water but cleavage does not occur (in my experiments; 
cf. Fischer), though some differentiation may take place without 
cleavage. It would appear, then, that conditions that so alter 
the permeability of the plasma and the vitelline membranes as 
to permit the outflow of the alveolar contents of the cortical layer 
initiate development, but that the normal continuation of devel- 
opment is dependent on other factors. 
In the normal fertilization of Nereis the stimulus of the sperm- 
atozoon causes the formation of the jelly and the perivitelline 
space long before it has penetrated the membrane; in fact pene- 
tration does not take place until 40 to 50 minutes after insemina- 
tion. However, mere contact of the spermatozoon with the mem- 
brane is apparently not sufficient; but actual attachment of at 
least a single spermatozoon is required; this is shown by the fact 
that the effective spermatozoon is not carried away from the 
membrane with the unsuccessful ones by the outflow of jelly. 
Yet the effect of the localized stimulus of the attached spermato- 
zoon is practically instantaneously effective over the entire 
