314 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 011 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. 



II. THE RELATIVE SENSITIVENESS OF FUNDULUS EMBRYOS 

 TO LOSS OF WATER 1 



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. 2 An accurate knowledge of 



1 These experiments were made in the summer of 1892, at Woods Hole. 



2 See Part I, p. 191. 



