Energy Exchange and Enzyme Development During Embryogenesis 521 



an arbitrary definition has certain shortcom- 

 ings, one of the most important being that 

 a great deal of what is commonly regarded 

 as differentiation is also embraced by it. 

 However, this may be just as well, for, in 

 normal development, growth and differentia- 

 tion usually proceed apace. Little is known 

 directly about the energetics of protein syn- 

 thesis in the embryo. The recent studies on 

 the incorporation of labeled amino acids into 

 the microsomes of tissue homogenates and 

 the association of this process with oxidative 

 phosphorylation, through synergistic action 

 between microsomes and mitochondria, are, 

 therefore, of fundamental importance to em- 

 bryologists (see Pollister, '54, for a review 

 of the literature). Energy is required for 

 peptide bond synthesis, and it may reason- 

 ably be expected that embryonic growth, in- 

 volving as it does the synthesis of protein, 

 will require energy expenditure on the part 

 of the embryo. 



Differentiation involves the progressive spe- 

 cialization of cells both structurally and 

 functionally. Its visible manifestations are the 

 form changes associated with cyto- and histo- 

 genesis, but, in addition to these important 

 macroscopic or microscopic changes, there 

 are events of equal significance at a molecu- 

 lar or macromolecular level of organization 

 (Porter, '54). The chemical changes in the 

 ontogeny of mitochondria or the elaboration 

 of specific chemical substances, for example 

 Nissl substance, actomyosin, or phosphatase, 

 represent differentiation just as surely as 

 do changes in cell shape or the develop- 

 ment of pigment or secretion granules. This 

 may appear obvious, but it is mentioned 

 to emphasize the fact that differentiation 

 involves synthesis of specific materials, as 

 does growth, and such syntheses require 

 energy. 



Much confusion and controversy have at- 

 tended consideration of the energetics of dif- 

 ferentiation (Needham, '31, '42; Tyler, '42; 

 Brachet, '50). One of the reasons for this 

 state of affairs stems from a number of 

 overly enthusiastic attempts to assess the 

 cost of differentiation in precise quantitative 

 terms from the results of experiments which 



in the sea urchin egg are haploid. RNA, since it is so 

 intimately linked with protein synthesis (Caspers- 

 son, '47; Brachet, '47) has also been suggested, pai-- 

 ticularly as a measure of cytoplasmic growth. Here 

 too, difficulties are encountered, for not all of the 

 RNA is located in the cytoplasm. Moreover. Herr- 

 man and Nicholas ('49) and Flexner and Flexner 

 ('51) have reported lack of correlation between 

 RNA content and cell volume. 



were not designed to yield such information. 

 Another difficulty arises from confusing what 

 may legitimately be considered as energy for 

 differentiation, i.e., energy for the synthesis 

 of specialized materials, with the so-called 

 "Organizational Energy," or OE, of Need- 

 ham's ('31) terminology. OE refers to a 

 hypothetical quota of energy which is in- 

 timately tied up with the organization of 

 the embryo and which should be released as 

 a measurable quantity when the embryo is 

 disorganized by cytolysis. If OE really exists, 

 it follows that the energy expenditure dur- 

 ing development, as measured directly by 

 heat production, should be significantly less 

 than that calcvdated from data on respiratory 

 exchange and the foodstuffs consumed dur- 

 ing development. Comparison of the results 

 of direct and indirect calorimetry has failed 

 to reveal such a quota of energy (Bohr and 

 Hasselbalch, '03), as have attempts to deter- 

 mine calorimetrically the evolution of heat 

 during cytolysis (Needham, '31). A later 

 study of the bee moth, Golleria mellonella 

 (Crescitelli, '35; Taylor and Crescitelli, '37), 

 indicates that energy expenditure during pu- 

 pation is higher when measured indirectly 

 from respiratory data than when determined 

 calorimetrically. But definite conclusions 

 from these observations cannot be drawn in 

 the absence of chemical data on the sources 

 of energy used during pupation. Smith's 

 ('46, '52) investigations of energetics during 

 the development of the rainbow trout show 

 conclusively that loss in total fuel value of 

 the embryo plus yolk is exactly equivalent 

 to the heat produced. 



Failure to find a definite quantity of en- 

 ergy that could be ascribed to OE has been 

 interpreted by some as indicating that dif- 

 ferentiation occurs without cost, but such a 

 conclusion is as unwarranted as is the view 

 that differentiation processes require a large 

 proportion of the embryo's total energy ex- 

 penditure. Butler ('46) has commented on 

 this as follows: "If an organism can syn- 

 thesize peptide bonds, it appears that it will 

 have no great difficulty putting together pro- 

 tein molecules of any degree of complica- 

 tion. The free energy must come from the 

 metabolic processes going on in the organ- 

 ism. The complete oxidation of a glucose 

 molecule to carbon dioxide and liquid water 

 yields approximately 700,000 cal. of free 

 energy per mol. This is of the order of mag- 

 nitude sufficient for the building up into pro- 

 teins of about a hundred amino-acid resi- 

 dues. Thus there is no outstanding difficulty 

 in accounting for the synthesis of living 



