SECT. 7] 



OF EMBRYONIC DEVELOPMENT 



993 



O wet weight 



©dry ,. 



B mgmsyocoagulable 

 protein dissappear- 

 ing perday: wet 

 weight: calc. from 

 Sakurao' 



The work of Bialascewicz and Mincovna will be considered in more 

 detail in Sections 9-9 and 11-3. 



The conception of an ontogenetic succcession of energy sources 

 has to reckon, however, with a few facts which do not easily fit it. 

 Perhaps the most difficult phenomenon to explain from this point of 

 view is the apparent combustion of carbohydrate exclusively by 

 mammalian embryos (Bohr) . 

 It must be admitted that the 

 evidence for this is slender, but 

 even if it were true, it would not 

 be surprising, as most Living cells 

 combust carbohydrate if they 

 can get it, and the continuous 

 perfusion system of viviparity 

 may provide such a supply. 

 Perhaps mammalian embryos 

 might be regarded as having 

 prolonged their carbohydrate 

 period to cover the whole of 

 their pre-natal life, and, if this were so, the peaks in basal metabolic 

 rate found by Wood; DuBois, and others on mammals shortly after 

 birth might be associated with maximum intensities of protein 

 combustion. 



Probably other substances besides protein, fat and carbohydrates 

 may be utilised to supply energy in some forms of life. For example, 

 the recent discovery by Heilbron of great amounts of spinacene, a 

 cholesterol-like substance, in selachian eggs, may lead to the solution 

 of the problem of the energy-source of these eggs. What they combust 

 has so far been quite unknown. 



Grafe in 1910 thought that there might be some connection be- 

 tween the period of carbohydrate utilisation in the chick's develop- 

 ment and the fact that at that time the most profound morphogenetic 

 changes were going on. And it has been suggested that the fat period 

 at the end might be associated with preponderance of change of size 

 over change of shape. But perhaps the time has not yet arrived for 

 correlations of this kind. Again, some connection may appear between 

 the succession of energy-sources in ontogenesis and the numerous 

 observations of susceptible stages in development, such as those of 



5 10 



ngrris./i protein combusted per day 



Fig. 261 



