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, to show 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 P fingers Archiv two years ago. 1 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 



i Pfliigers Archiv, Vol. LV, p. 530. 



