SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 



68 1 



or no change in the respiration until the external gills appear, at 

 which stage there is a third increase of oxygen-uptake. The succeeding 

 period is again marked by a uniformity of rise." The three critical 

 points in amphibian development, then, according to Parnas & 

 Krasinska, are (i) gastrulation, (ii) formation of medullary plate, 

 neural groove, etc., and (iii) appearance of external gills. These 

 relations can best be seen on the increment curve in Fig. 132, where 

 the breaks at gastrulation and the formation of the neural groove 

 can be seen well. 



150 



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50 



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Hours 

 Fig. 132. 



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Parnas & Krasinska considered the question of whether their 

 sudden increases in respiration could be accounted for by changes 

 in the surface-volume relation or other causes of non-metabolic 

 origin. At the neurula stage, for instance, when the spherical form 

 of the embryo is abandoned, a greater oxygen consumption might 

 be supposed to be due to the consequent increase of surface. But 

 they concluded that the operation of such factors did not account 

 for their results, since gastrulae respired no more in pure oxygen 

 than in air. 



Parnas & Krasinska suggested as the cause for the low respiratory 

 quotients found by Bialascewicz & Bledovski processes in which 

 oxygen was combined in the materials of the cells under construction 

 — the same explanation as Meyerhof had already advanced. If they 

 were indeed accurately measuring the carbon dioxide output of the 



