994 



ENERGETICS AND ENERGY-SOURCES [pt. iii 



Stockard and of Parnas & Krasinska. This work has brought out with 

 exceptional clearness the fact that development may be discontinuous, 

 and in all cases passes through critical stages when disastrous effects 

 will follow an interference innocuous at other times. The beginning 

 of gastrulation is such a critical stage. Stockard says, "The present 



1500r3000 



1000 



100 



2000 



-1000 



O Fiske g^Boyden 

 © Needham 

 a Sznerovna 



PERIOD or 



carbohydrate: 



combustion 



Days 



-JO 



2 3 4 5 



10 11 12 13 14 15 It] 17 18 19 20 21 



Fig. 262. 



extremely crude state of our knowledge of the chemistry of develop- 

 ment will permit of no . . . satisiactory statement of the principles 

 underlying differences in developmental rate". Critical stages in 

 development may turn out to be associated with changes from one 

 type of substance to another type as a source of energy. An inter- 

 mediate link in the chain of events would be the rapid growth-rate 

 of one or more organs, leading to a teratological result if development 

 was at that moment interfered with. 



The ultimate nature of the succession of energy sources presents 

 a problem of some interest. It is possible that carbohydrate is first 

 combusted because it requires no preparation. Proteins must be 

 deaminated, fats must be desaturated, and probably the embryo in 

 its earlier stages cannot do either of these things, but, on the other 

 hand, glucose lies ready for use, and it is significant that what is then 



