EFFECTS ON EGGS OF FRESH WATER PULMONATES 445 
cases in which the three substances have been sharpty separate 
the clear protoplasm afterward diffuses slowly into the gray and 
yellow zones, and if some time elapses between the centrifuging 
and the first cleavage, the clear zone ma}^ be largely diffused 
through the gray and yellow ones before the new division wall 
forms; but when centrifuging occurs just before the cleavage this 
readjustment does not take place so fully. ^ 
On the other hand it is very probable that there is an increasing 
differentiation of the ooplasm between the maturation and the 
first cleavage. This is indicated by the fact, among others, that 
all experiments on the cleavage stages of gasteropods indicate that 
as early as the first cleavage differentiation has progressed so far 
that the development of isolated blastomeres is strictly partial, 
(Crampton 1896). That there is a progressive differentiation 
of the cytoplasm in the period between maturation and cleavage 
is shown also by the work of Wilson (1903) and Yatsu (1904) on 
Cerebratulus. I have not found it possible to apply the methods 
used by these investigators to the study of the eggs of these pul- 
monates, protected as they are by resistant, elastic capsules, but 
the results of my experiments with centrifugal force are in full 
accord with the works mentioned, and these results find a ready 
explanation if there is a progressive increase in the differentiation 
of the egg substance. 
In conclusion then, it seems probable that the injurious effects 
of centrifuging, in the latter part of the period before cleavage, 
are due principally to the greater differentiation of the ooplasm 
at this period. This conclusion suggests a possible means of har- 
monizing the discordant results so far obtained in centrifuging 
the eggs of different animals, as for example, those of echinoderms 
and ascidians. If the injurious effect of centrifuging is propor- 
tional to the degree of differentiation of the ooplasm, these appar- 
ently discordant results may find a ready explanation. 
6. Finally these experiments show that the differently colored 
substances of these eggs are not ''organ forming" in the sense 
^ In eggs which have been killed by centrifuging, such diffusion of the clear 
material into the yellow and gray zones does not take place, but the three 
zones remain distinct indefinitely. 
