422 STUDIES IN GENERAL PHYSIOLOGY 
through lack of oxygen, which in their turn are the cause 
of the suspension of life-phenomena. This has been proved 
for the process of cleavage in the Ctenolabrus egg. The 
cleavage-cells of Ctenolabrus are dissolved again and fuse 
together when oxygen is removed. These changes are not, 
however, an evidence of death, for as soon as such a fused 
blastoderm is again exposed to air it begins to divide anew. 
On the other hand, these molecular changes are sufficient to 
hinder cleavage. The cleavage-cells of the Arbacia egg 
seem to suffer similarly in the lack of oxygen, although the 
changes are much less marked. We find that here also 
cleavage is impossible without oxygen. Yet lack of oxygen 
does not bring about the same sort of molecular changes in 
the Fundulus egg as in the Ctenolabrus egg, and corre- 
sponding with this difference cleavage may also go on with- 
out oxygen for many hours in Fundulus. 
It is also possible that such molecular changes as are 
brought about by the lack of oxygen in the cell are also the 
cause of the cessation of other life-phenomena; for example, 
the beat- of the heart (and the activity of the respiratory 
center). We thus find that in Ctenolabrus, where the first 
cleavage-cells suffer such profound structural changes 
through lack of oxygen, the heart of the embryo comes to a 
standstill very rapidly and suddenly through lack of oxygen 
before a marked decrease has taken place in the frequency 
of the heart-beats; while the heart of the Fundulus, whose 
cells suffer no such structural changes, continues to beat for 
many hours without oxygen. Since the chemical energy 
set free in the cells must first be converted into molecular 
energy in order to bring about the physiological function, it 
is clear, a priori, that not only a decrease in the supply of 
the chemical energy, but any structural change which ren- 
ders impossible the conversion of chemical energy into the 
molecular energy necessary for the activity of the tissue, 
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