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 c.c. 

 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 



