J. R. GREGG 259 



is not known whether, under the stress of anaerobiosis, hybrid 

 gastrulae are less able than jR. pipiens controls to maintain their 

 total stores of esterified phosphorus, but they do not appear 

 to expend more phosphate bond energy under anaerobiosis 

 than do R. pipiens controls. Their only known tissue-localized 

 metabolic difference from normal embryos is the deficient rate 

 at which acid breis of their dorsal tissues will liberate inorganic 

 phosphorus. They probably synthesize deoxyribosenucleic acid 

 at the normal rate until they are in Stage H14, after which there 

 is a fixation of the steady-state level at which this compound is 

 maintained. 



Unfortunately, it is difficult at the present time to relate the 

 biochemical features of hybrid embryos to their developmental 

 peculiarities. It is tempting to suppose that the lowered rates at 

 which they liberate energy from glycolytic processes is causally 

 connected with their delay in beginning to gastrulate, with the 

 slowness with which epiboly proceeds, or even with the com- 

 plete morphogenetic failme of mesoderm which so effectively 

 disorganizes gastrular movements. For it is plausible, at least, to 

 suppose that morphogenetic processes — tissue movements, cell- 

 shape changes, and the rest — are endergonic processes, and that 

 when an unimpaired supply of energy is not forthcoming such 

 processes must necessarily slow down or fail altogether. But it is 

 also possible that the morphogenetic peculiarities of hybrid em- 

 bryos and their metabolic ones are jointly caused. We do not know 

 what alterations of fine structure may occur in a hybrid as a re- 

 sult of its unusual parentage. It is easy to imagine, however, that 

 they are of such a kind that neither morphogenetic changes nor 

 metabolic processes can occur normally, even in the presence of 

 a nomial energetic coupling between the latter and the former. 

 Clarification of these questions must await further thought and 

 investigation. 



REFERENCES 



Earth, L. G. 1946. Studies on the metabolism of development. /. Expth 

 Zool, 103, 463-86. 



