Energy Exchange and Enzyme Development During Embryogenesis 533 



There is an additional reason for ques- 

 tioning the validity of considering the main- 

 tenance requirements of a developing 

 embryo as identical with those of a blocked 

 embryo. It is entirely conceivable that the 

 cost of maintenance for a developing system 

 would be significantly greater than for a 

 non-developing one. The rate of chemical 

 turnover in the unblocked embryo is cer- 

 tainly more extensive, the deterioration, 

 through activity, of various enzymes is prob- 

 ably greater, and "maintenance protein 

 synthesis," as well as other synthetic proc- 

 esses, is undoubtedly going on at a faster 

 rate. It seems premature, therefore, to at- 

 tempt to fix either the absolute or relative 

 costs of maintenance and development, either 

 as a total process or in terms of its sub- 

 divisions, from the kind of data available at 

 the present time. 



On the question of the existence of differ- 

 ent respiratory mechanisms concerned with 

 maintenance and development, it can be said 

 that there appears to be little doubt that 

 X-sensitive and X-insensitive fractions of 

 respiration are found in the developing sys- 

 tem. But the relationship of such systems 

 to development on the one hand and to 

 maintenance on the other is more obscure. 

 One fundamental difficulty in using data 

 derived from studies with inhibitors, in 

 attempting to assess the relative amounts of 

 energy exchange concerned with mainten- 

 ance or with development, lies in the fact 

 that the magnitude of the inhibitor-insensi- 

 tive fraction of respiration, svipposedly con- 

 cerned with maintenance, varies widely and 

 seems to depend upon the particular inhib- 

 itor used. Thus, Fisher and Henry ('44) 

 found that sulfanilamides inhibit cell di- 

 vision at concentrations that reduce respi- 

 ration by no more than 45 per cent, and the 

 concentration of penicillin required to block 

 cleavage completely was found to have ab- 

 solutely no effect on respiration (Henry and 

 Henry, '45). The very multiplicity of the 

 agents represented by X, and the fact that 

 in certain cases X-sensitive and X-insensitive 

 fractions can both be inhibited by the same 

 substance, permit certain reservations as to 

 the validity of considering the various frac- 

 tions of respiration as proceeding through 

 different mechanisms. 



ENERGY SOURCES DURING DEVELOPMENT 



The Ontogenetic Sequence: Carbohydrate, 

 Protein, Fat. Needham ('31, '42) has gathered 

 together an impressive body of data to es- 



tablish the fact that the chick embryo uses 

 the primary energy sources during develop- 

 ment in the order — carbohydrate, protein, 

 fat. Evidence for this has been derived from 

 three sources: (1) chemical analysis of the 

 egg contents and embryo at various stages 

 of development, (2) respiratory quotient data 

 for the egg and isolated embryo, and (3) 

 estimations of the amovmt and nature of 

 nitrogenous wastes produced by the embryo 

 and extra-embryonic structures. 



The results of in vitro studies are in 

 harmony with the concept that carbohydrate 

 is the energy source of major importance for 

 the early chick embryo (Dickens and Gre- 

 ville, 33a,b; Philips, '41, '42). The more 

 recent work of Spratt ('48, '50) points in 

 the same direction, for he found that differ- 

 entiation of chick blastoderms, when culti- 

 vated in synthetic media, would not occur 

 unless glucose was present. Essentially 

 similar results have been obtained by Taylor 

 and Schechtman ('49). 



Protein metabolism occurs throughout de- 

 velopment, with most of the protein absorbed 

 by the embryo being used for growth. 

 Protein utilization for catabolic purposes 

 begins to be significant on about the fifth or 

 sixth day and is maximal on the eighth or 

 ninth day. Conversely, anabolic protein util- 

 ization is high during the first few days of 

 development and minimal on the eighth or 

 ninth day. Subsequently, the rate of absorp- 

 tion of protein for growth rises gradually to 

 a new peak reached on the fifteenth day. 

 Novikoff and Potter ('48) have shown that 

 the pentose nucleic acid content of the em- 

 bryo fluctuates in phase with the processes 

 outlined above.* 



Fat is the predominant energy source in 

 avian development. About 60 per cent of 

 the total fatty acid present in the egg at the 

 beginning is combusted, and from this the 

 embryo derives over 90 per cent of its caloric 

 yield. Fat uptake, leading to combustion, 

 begins at about the fourth or fifth day, and 

 continues to increase in intensity, paralleling 

 the increase in size of the embryo, through- 

 out the remainder of the incubation period. 



In some few other embryos, the ontoge- 

 netic sequence shown for the chick seems to 



* Novikoff and Potter compare their curve for 

 PNA with data from Needham ('31) showing pro- 

 tein content of the embryo in milligrams per cent 

 dry weight. Had they used instead the intensity of 

 protein absorption by the embryo (which is perhaps 

 a better measure of protein metabolism) the paral- 

 lelism of their results with those summarized by 

 Needliam would have been even more striking. 



