PHYSIOLOGICAL Errscts or Lack oF OxyaEen 3875 
supported by the fact that the eggs of Fundulus in which 
such structural changes do not occur in the absence of 
oxygen even after twenty-four or more hours, continue to seg- 
ment for more than twelve hours in the absence of oxygen. 
What we are going to show for cleavage holds also for 
other life-phenomena, for example, the activity of the heart. 
We find that the heart of the Ctenolabrus embryo comes to 
a standstill so quickly after the oxgen has been withdrawn 
that it is impossible to think that the source of the energy 
for the beating of the heart has given out, while the heart 
of the Fundulus embryo continues to beat for many hours 
under similar conditions. It is possible that in this case 
also structural changes occur which are similar to those 
which we are able to observe directly in the cleavage-cells. 
These changes render impossible the transformation of 
chemical energy into mechanical energy in the contraction 
of the heart of the Ctenolabrus embryo. 
Such molecular changes as manifest themselves by struc- 
tural changes can be brought about just as well through 
processes of reduction due to the lack of oxygen as through 
the injurious compounds which may be formed in the absence 
of oxygen. 
The facts which we obtained by the biological study of 
the effects of lack of oxygen may also again be of importance 
to the physiological chemist. We shall meet with some 
facts in this paper which will serve to illustrate the view of 
Bunge that in all probability the greater part of the energy 
necessary for considerable work of the muscles is furnished 
through processes of oxidation (and not through processes of 
hydrolysis). The frequency of the heart-beat of the Fundu- 
lus embryo decreases steadily during the period during 
which oxygen is withdrawn, until it reaches the minimum (of 
about twenty beats per minute), when all the oxygen has dis- 
appeared. But the heart can beat for ten hours at this rate 
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