PHYSIOLOGICAL EFFECTS oF Lack oF OxyGEN 377 
different, however, when the function which we are study- 
ing does not cease immediately. We are then unable to 
say whether the transitory persistence of the function shows 
that not all of the oxygen has been driven out, or that 
the given function is not directly dependent upon oxygen. 
Things of this sort confront us when we attempt to decide 
whether cleavage is possible without oxygen. We find 
that when sea-urchin eggs are placed in an Engelmann 
chamber immediately after fertilization, and we begin to 
pass hydrogen through it, the eggs not only cleave into 
two, but often even into four, cells; but this cleavage occurs 
within the first fifty or eighty minutes after fertilization, and 
it might be thought that it takes a longer time than this to 
drive out all the oxygen. To be certain on this point in 
such cases, I made use of the following procedure: I in- 
troduced the eggs in which I wished to study the depend- 
ence of cleavage upon oxygen into an Engelmann chamber 
which was kept on ice. Hydrogen was then passed through 
the apparatus. The low temperature inhibits cleavage. In 
order to discover when I could take the eggs off the ice, and 
know that the objection could no longer be raised that the 
eggs still contain oxygen, I introduced a second gas-chamber 
into the circuit. This contained eggs of the same culture, 
and through it I passed the same current of gas as that 
which went through the first. The second, control, chamber 
was not puton ice. As long asa trace of cleavage continued 
in this control chamber, there was reason to suspect that not 
all the oxygen was driven out. As soon, however, as cleavage 
ceased, it seemed reasonable to assume that, even though not 
all the oxygen had been driven out of this chamber, the por- 
tion which remained behind was no longer sufficient to start 
cleavage. It must be remembered, however, that the eggs 
kept on ice had not lost as much oxygen as the control eggs 
during this time, for oxidation did not occur as rapidly in the 
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