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734 STUDIES IN GENERAL PHYSIOLOGY 
centage of mature eggs is determined, this percentage is 
subject to the greatest variations. The cause of these 
variations was soon discovered. For it was found, where 
the eggs lie together in a heap maturation occurs slowly, 
but where they lie in a thin layer, maturation occurs quickly. 
This fact suggested the importance of oxygen for matura- 
tion. Where the eggs lie in a heap the appropriation of the 
oxygen by the superticial layers of eggs prevents the diffu- 
sion of the oxygen to those lying deeper. 
Experiments were now made in which the oxygen of asmall 
flask containing a small amount of sea-water was replaced 
by hydrogen. When, in such experiments, all the oxygen 
was entirely removed maturation did not occur in any, or at 
least the majority of the eggs, in spite of the presence of 
the hydroxyl ions in the sea-water. There are, therefore, at 
least two substances in sea-water which cause or accelerate 
maturation, oxygen and hydroxyl ions. Possibly other con- 
stituents of the sea-water are also concerned in the process, 
but NaCl, Ca, and K have apparently no beneficial effect upon 
maturation.’ 
It seems, therefore, that the absence of oxygen and 
hydroxyl ions in the ovaries belongs to the conditions which 
inhibit maturation of the eggs in the ovary. 
IV. THE PROLONGATION OF THE LIFE OF THE UNFERTILIZED 
STARFISH EGG BY THE PREVENTION OF MATURATION 
We have shown above that the mature eggs of a culture 
of unfertilized starfish eggs die within a short time (which 
decreases with an increase in temperature), while the imma- 
ture eggs remain alive a relatively long time. It was 
necessary now to show that when the maturation of a culture 
of unfertilized egg of Asterias is prevented artificially, the 
1 Professor Whitman informs me that the maturation of the eggs of Clepsine 
does not begin until after they are laid. Possibly the oxygen contained in the water 
is in this case also a necessary condition for maturation. 
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