314 STUDIES IN GENERAL PHYSIOLOGY 



of oxygen. The young fish which had just hatched from 

 the egg were still less resistant than tin- 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 cmliri/o. for c.i-dmjile, has just as </ood, 

 or ere n a better chance, for continuing its development irhen 

 if has remained for all this lime in <in o.r// ; /en menu in than 

 irhen if has sfieiit only the last forfi/-ei</lif hours in flic 

 raennni. 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 euibrvo 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 jnst been fertilized may still lie 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 fortv- 

 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. 



II. THE RELATIVE SENSITIVENESS OF FUNDULUS EMBRYOS 



TO LOSS OF \VATER 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 Parti, p. 191. 



