538 Energy Exchange and Enzyme Development During Embryogenesis 



much as 40 per cent of the total respiration 

 of the embryo is absorbed by the standard 

 deviation. Wills' data for R. pipiens (Table 

 20) actually indicate more nitrogen present 

 at the end of development than at the be- 

 ginning, and it can be shown that the differ- 

 ence is statistically significant! 



The yolk of amphibian eggs contains a 

 relatively high proportion of phosphoprotein. 

 The phosphate linkages are present in the 

 form of high-energy bonds, and Harris ('46) 

 discovered a special phosphoproteinphospha- 

 tase capable of releasing phosphate without 

 prior action of protease. The mechanism of 

 phosphate transfer from yolk is complex and 

 involves ATP, other proteins, and several 

 enzymes (Barth and Jaeger, '47a,b; '50a,b; 

 Barth and Barth, '51). The high-energy 

 bonds of phosphoprotein apparently represent 

 a relatively insignificant store of energy, 

 however, for it has been calculated that the 

 net amount available is only 0.15 per cent of 

 the total energy consumption during develop- 

 ment (L0Vtrup, '53a). 



Relatively little work has been done on 

 fat metabolism during amphibian embryo- 

 genesis. That some fat is burned is evident 

 from the analytical data of Bialascewicz and 

 Bledovski, Barthelmy and Bonnet, and Parnas 

 and Krasinka (cited from Needham, '31). 

 Furthermore, Atlas ('38) has concluded, from 

 a comparison of the loss in dry weight and 

 oxygen consumption of R. pipiens, that some 

 fat is used as an energy source. L^vtrup's 

 findings also point in the same direction. The 

 majority of these workers agree that fat 

 utilization takes place predominantly during 

 the latter part of development, although there 

 may be some indication that it begins earlier 

 than was formerly believed (L0Vtrup, '53a). 



In conclusion, then, it may be stated that 

 the exact order in which energy sources are 

 used during amphibian ontogeny is unknown. 

 There actually may be no sequence at all in 

 the sense that it can be demonstrated in the 

 chick, but there is some indication that pro- 

 tein combustion precedes that of carbohy- 

 drate in early development, and that after 

 gastrulation oxidation of all three foodstuffs 

 occurs. 



Factors Responsible for Changes in Energy 

 Source Utilization. A problem of considerable 

 interest concerns the mechanism by which 

 the embryo shifts from one class of foodstuffs 

 to another during development. It might ap- 

 pear reasonable to assume that one energy 

 source would be used preferentially until its 

 concentration in the egg became so reduced 

 that it could no longer be mobilized in ade- 



quate amounts; then a second energy source 

 would be called upon. However, most of the 

 information available at present from experi- 

 mental studies suggests that the change to 

 a second energy source is not brought about 

 by the exhaustion of the one previously used. 

 It has been shown that the chick embryo 

 will enter upon the "protein phase" of me- 

 tabolism even though carbohydrate is still 

 present or when the carbohydrate supplies 

 are artificially increased by injection of glu- 

 cose into the egg. There is also evidence that 

 failure to use an energy source is not the 

 result of lack of necessary enzymatic machin- 

 ery. Dickens and Greville ('33b) have foim:d 

 that chick embryos, in vitro, can burn protein 

 leading to ammonia production during the 

 period of development when the normal fuel 

 is carbohydrate. Likewise, Needham ('32b) 

 has shown that early chick embryos will 

 bum protein when treated with fluoride. The 

 grasshopper embryo, during the stage when 

 90 per cent of its energy yield is derived 

 from fat oxidation, will burn glucose when 

 it is supplied (Bodine and Boell, '36b), and 

 the embryo can be induced to burn protein 

 by treatment with dinitrophenol (Bodine and 

 Boell, '38). It thus appears that the sequence 

 of energy source utilization depends upon 

 conditions in the embryo itself, but what 

 these are is at present unknown. 



The question as to whether various devel- 

 opmental processes may require special or 

 unique sources of energy, different from those 

 used for maintenance, cannot be definitely 

 answered. But it may be suggested, on the 

 following grounds, that they do not. It is be- 

 coming increasingly clear that high-energy 

 phosphate bonds in such compounds as ATP 

 represent the immediate energetic source for 

 development, and it seems likely that the ox- 

 idative processes required to generate phos- 

 phate bond energy would be the same no 

 matter what the ultimate destiny of the en- 

 ergy stored in them may be. The real prob- 

 lem for the student of development is not 

 how high-energy bonds are synthesized, but 

 rather how the energy released from them is 

 directed to bring about specific end results in 

 the embryo. On this question ignorance is 

 complete. 



RESPIRATORY MECHANISMS 



The Embden-Meyerhof, Citric Acid Cycle, 

 Warburg-Keilin System. In the breakdown of 

 carbohydrate to carbon dioxide and water, in 

 the systems that have been most thoroughly 

 investigated — yeast and skeletal muscle — the 



