PHYSIOLOGICAL Errrots or Lack or Oxyaren 883 
when all the exhaustible oxygen has been removed so that 
cell-division is no longer possible. The energy necessary 
for these changes must probably therefore be obtained from 
processes of hydrolysis. 
One might be tempted to believe that the nucleus could 
continue to divide without oxygen, while the cell remains 
undivided—a phenomenon I discovered in sea-urchin eggs, 
when they are brought into sea-water of a certain concentra- 
tion. In such eggs the number of nuclei steadily increases, 
but no cell-division occurs. But such phenomena certainly 
do not take place in the absence of oxygen. Eggs were 
freed from oxygen by passing hydrogen over them for two 
hours while on ice. They were then exposed for one hour 
to room temperature, while the flow of hydrogen was not 
interrupted. No cleavage had occurred. The eggs were 
then killed and sectioned. It was impossible to find more 
than one nucleus in these eggs; this was, however, in a 
number of instances undergoing mitosis. The experiment 
was repeated with the same result. One mitotic division 
may therefore occur without oxygen, but no more. 
If eggs which have been freed from oxygen for a suffi- 
ciently long time while on ice, and which have shown no 
evidence of cell-division when exposed to hydrogen for an 
hour at room temperature, are exposed to the air, they all 
divide in the course of thirty to fifty minutes. But they do 
not then first divide into two cells and later into four, but 
immediately into four—occasionally into three or five cells. 
This also occurs when a strong stream of hydrogen is sent 
through the gas-chamber for three and a half hours at a low 
temperature before the experiment is begun—under condi- 
tions therefore when, in all probability, all of the free oxygen 
has been removed from the eggs. Two divisions of the 
nucleus therefore always occur—one in the hydrogen, and 
one after the admission of air—before the first cell-division 
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