B14 STUDIES IN GENERAL PHYSIOLOGY 
of oxygen. The young fish which had just hatched from 
the egg were still less resistant than the embryos. 
These experiments show that the sensitiveness of the 
embryo to lack of oxygen increases with development. This 
increase is very great at the beginning of development, so 
that a four-day old embryo, for example, has just as good, 
or even a better chance, for continuing its development when 
it has remained for all this time in an oxygen vacuum than 
when it has spent only the last forty-eight hours in the 
vacuum. This apparently paradoxical result is readily 
explained when we assume that the cells which are formed 
from the egg-cell during the first stages of cleavage are 
different chemically from the cells of the embryo which are 
formed later, so that the latter go to pieces more easily in 
lack of oxygen than the former. For this reason an egg 
which has just been fertilized may still be capable of develop- 
ment when it has spent four days in an oxygen vacuum, 
because it has developed only to the point of the formation 
of a blastoderm; while an egg which is introduced into the 
vacuum after the formation of the embryo dies after forty- 
eight hours. We saw also that development can go on for 
about fifteen hours in the oxygen vacuum, especially in the 
first twenty-four hours after fertilization. Whether the 
conclusion can be drawn from this that cleavage can go on 
without oxygen, or that the oxygen was not completely 
absorbed in our experiments, must be determined by further 
experiments. 
Il. THE RELATIVE SENSITIVENESS OF FUNDULUS EMBRYOS 
TO LOSS OF WATER’ 
The development of the form of an embryo is a function 
of processes of cell-division and growth. Both classes of 
processes are, as in plants, so also in animals, probably a 
function of osmotic processes.” An accurate knowledge of 
1 These experiments were made in the summer of 1892, at Woods Hole. 
2See Part I, p. 191. 
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