KELATIVE SENSITIVENESS OF FISH EMBRYOS 313 

 ment would have gone in normal sea-water. These eggs 



O OO 



lost their power of development in the oxygen vacuum 

 usually after about forty-eight hours. The maximum that I 

 observed in one case was a continuation of development 

 after a residence of fifty-five hours in the oxygen vacuum. 

 An egg which is introduced into the oxygen vacuum twenty- 

 four hours after fertilization loses its power of develop- 

 ment much more quickly than an egg which is exposed 

 to the same degree of lack of oxygen immediately after 

 fertilization. 



If embryos are introduced into the oxygen vacuum forty- 

 eight hours after fertilization, the retardation of develop- 

 ment becomes more apparent. The formation of the embryo 

 has already begun in these eggs, and the next elements in 

 the further development of the egg would be the formation 

 of pigment and the beginning of the circulation. Pigment 

 is indeed formed, though less than normally, but no circula- 

 tion is established. After remaining for thirty-two hours in 

 the oxygen vacuum, these eggs had lost their power of 

 development also. 



Seventy-two hours after fertilization the circulation is 

 fully developed in an embryo, when it is left in normally 

 aerated sea-water ; when at that time the embryo is placed 

 into the oxygen vacuum, in about seven hours the heart- 

 beat becomes very weak. When such an egg was returned to 

 normal sea-water, however, the heart of the embryo began to 

 beat again vigorously almost immediately, at first slowly, 

 but then with so rapid an increase in the rate that the num- 

 ber of beats became relatively great in a few minutes. These 

 eggs lost their power of development in lack of oxygen in 

 about twenty-four hours after introduction into the oxygen 

 vacuum. In no case did such an egg recover when it had 

 remained for forty-eight hours in the oxygen vacuum. The 

 older the embryo, therefore, the more sensitive it is to lack 



