566 STUDIES IN GENERAL PHYSIOLOGY 
twenty-four hours. The third stage of development begins 
with the establishment of muscular activity, especially the 
heart-beat, about seventy-two hours after fertilization. 
If newly fertilized eggs be exposed to a partial oxygen 
vacuum, the development may go on for some time (about 
twenty-four hours). The eggs, however, may remain alive 
in the oxygen vacuum for four days. If after that time 
they are put back into normal sea-water, they develop into 
normal fish, which hatch in due time. If we put an embryo 
which is three days old into the same oxygen vacuum, it 
loses its power of development within twenty-four hours. 
The older the embryo, the more deleterious is the lack of 
oxygen. This is comprehensible only on the assumption 
that the morphological differentiation is accompanied or 
preceded by changes in the chemical constitution of the 
embryo. 
The same result was obtained in experiments in which 
the concentration of sea-water was raised by the addition of 
NaCl, but curiously enough in this case the younger embryo 
was more sensitive to an addition of NaCl to sea-water than 
the older embryo. An addition of 5 g. of NaCl to 100 cc. 
of sea-water did not prevent the development of the Fundulus 
egg, but an addition of 10 g. of NaCl to 100 c.c. of sea- 
water prevented the formation of an embryo. Newly fer- 
tilized eggs began to segment in such a solution, but stopped 
very soon and lost their power of development permanently 
within from six to ten hours. A germ that was allowed to 
develop during the first twenty-four hours in normal sea- 
water withstood much better a solution of sea-water to 
which 10 per cent. of NaCl had been added. In such a 
solution it could go on with its development for several days; 
in some cases as long as ten to fourteen days. An embryo 
which had been allowed to develop in normal sea-water until 
its circulation was established (third or fourth day) was even 
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