RELATIVE SENS([TIVENESS OF FisH Emsryos' 3813 
ment would have gone in normal sea-water. These eggs 
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 
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