PHYSIOLOGICAL EFFECTS oF Lack oF OxyGEN 373 
rendered difficult by the fact that many animals soon die 
without oxygen, and I believe that this fact explains why 
this field has not thus far been the subject of more study. 
Still the duration of life is in most cases sufficiently long to 
demonstrate a series of such changes. Copepods are, for 
example, exceedingly sensitive to lack of oxygen; I know no 
other cold-blooded animals that die so rapidly without 
oxygen. Yet it is possible, as we shall see, toshow definitely 
even in these animals that lack of oxygen affects their helio- 
tropic sense in a most remarkable way; when deprived of 
oxygen negatively heliotropic Copepods become positively 
heliotropic. 
Another consideration shows the importance of such inves- 
tigation. Physiological chemistry alone may suffice to dis- 
close the general sources of energy in animals. But the 
question as to how chemical energy is converted into the 
physiological activities of muscles, glands, etc., can of course 
not be answered by purely chemical researches. Molecular 
physiology must here bridge the chasm between the chemi- 
cal changes and the outwardly manifested physiological 
activities of the organs. A complete understanding of the 
energetics of animals is not possible so long as we have no 
conception of the molecular changes which are brought 
about through processes of oxidation. It therefore seemed 
of importance to see whether such changes manifest them- 
selves when oxygen is taken away. In this way arose the 
experiments detailed here on cleavage without oxygen, which 
I began three years ago, and which I discussed in a short 
note which appeared in Pfligers Archiv two years ago.' I 
directed my attention to processes of segmentation because I 
considered these phenomena especially favorable for obtain- 
ing facts for a molecular physiology. 
When we find that a physiological function is impossible 
1 Pfliigers Archiv, Vol. LV, p. 580. 
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