534 Energy Exchange and Enzyme Development During Embryogenesis 



hold, but much of the evidence is derived 

 from respiratory quotient data. Fiske and 

 Boyden ('26) showed long ago that respir- 

 atory quotients can be misleading as indica- 

 tors of the kinds of metabolites burned, for 

 the combustion of protein will yield res- 

 piratory quotients of approximately 1, 0.8, 

 or 0.7, depending upon whether the nitrog- 

 enous end product is ammonia, urea, or 

 uric acid. Moreover, with some methods of 

 respiratory quotient determination, three 

 different samples of living material are 

 required, and, unless due consideration be 

 given to assure uniformity of the eggs or 

 embryos used, distorted values will result. 



In the grasshopper embryo, the sequence 

 of energy source utilization seems to be the 

 same as in the chick (Slifer, '30; Boell, '35; 

 Hill, '45). The phase characterized by pre- 

 dominant carbohydrate metabolism is very 

 short, however, lasting not more than a day 

 or two. It also appears that protein occupies 

 a relatively insignificant role as an energy 

 source, but that some protein is broken down 

 is indicated by the accumulation of urates 

 in the embryo (Bodine, '46). Approximately 

 75 per cent of the total oxygen consumption 

 can be accounted for as fat oxidation. How- 

 ever, during the first two weeks of develop- 

 ment, fat combustion accovmts for no more 

 than a third of the embryo's respiration, 

 while during the post-diapause period almost 

 90 per cent of the total oxygen consumption 

 is at the expense of fat. Fat has also been 

 shown to be the raajor energy source in a 

 number of other insects (references in Boell, 

 '35; Crescitelli, '35; Needham, '42; Ludwig, 

 '50a, b), and in the silkworm the utilization 

 of energy sources occurs in the same sequence 

 as in the chick and grasshopper (Needham, 

 '42). 



A progressive change in respiratory quo- 

 tient, with high values initially and lower 

 ones, characteristic of fat oxidation, predom- 

 inating in later development, has been re- 

 ported for Fundulus (Amberson and Arm- 

 strong, '33), Carcinus (Needham, '33), 

 Urechis (Horowitz, '40), and Trichurus 

 (Nolf, '32). Thus the ontogenetic sequence — 

 carbohydrate, protein, fat — -may operate in 

 these embryos also, but, as was pointed out 

 above, respiratory quotient data alone are 

 not adequate proof. 



The Fish Embryo. Both in 1931 and 1942, 

 Needham noted that the ontogenetic sequence 

 — carbohydrate, protein, fat — apparently did 

 not hold in certain cases. Echinoderms and 

 amphibians represented outstanding excep- 

 tions. Smith's ('46, '52) thorough studies of 



the energetics of the rainbow trout, Salmo 

 irideus, indicate that this form also repre- 

 sents an exception. Smith has shown that 

 the amount of carbohydrate present at any 

 one time and consumed during particular 

 periods of development is so small in pro- 

 portion to the total protein and fat combus- 

 tion that carbohydrate as an energy source 

 can be ignored. Carbohydrate combustion is 

 limited to three short periods of develop- 

 ment: (1) immediately after gastrulation, 

 (2) at the time of hatching, and (3) at the 

 onset of starvation after exhaustion of the 

 yolk supplies. In the Atlantic salmon also, 

 carbohydrate metabolism seems to be rela- 

 tively unimportant, for, according to Hayes 

 and Hollett ('40), glycogen is absent in early 

 development and the amount of glucose pres- 

 ent is exceedingly small. It may be remarked 

 that the failure of Hayes and Hollett to find 

 glycogen in the early embryo has been at- 

 tributed to faulty technique. In Salmo solar, 

 Daniel ('47) reports that glycogen is present 

 at the beginning of development, but he 

 agrees with Smith and Hayes and Hollett 

 that the quantity is small. 



In the rainbow trout, protein and fat are 

 used simultaneously and account for 99 per 

 cent of the total caloric yield during develop- 

 ment. There is some indication that the 

 percentage of protein burned is higher dur- 

 ing early development (39 to 42 per cent) 

 than later on (27 to 30 per cent), and it 

 seems clear that consumption of water-sol- 

 uble phosphatide fat precedes that of glycer- 

 ide fat. During early development, Smith's 

 analyses show that ammonia may account 

 for as much as 95 per cent of the total 

 nitrogen excreted. Possibly the initial high 

 respiratory quotients reported for Fundulus 

 (Amberson and Armstrong, '33) may have 

 been due to protein combustion with am- 

 monia as the end product. 



The Sea Urchin Embryo. Although res- 

 piratory quotients of unity were found for 

 the sea urchin egg immediately after ferti- 

 lization by some of the earlier workers, it 

 appears now that these values were abnor- 

 mally high and were biased by the inclusion 

 of non-respiratory carbon dioxide. Runnstrom 

 ('33) showed that an acid is liberated from 

 sea urchin eggs at fertilization of sufficient 

 strength to release carbon dioxide from bi- 

 carbonates in sea water. Accordingly, Runn- 

 strom ('34) obtained "respiratory quotients" 

 of 2, or even higher, during the first 15 min- 

 vites after fertilization. 



The origin of the acid is of interest al- 

 though it is only peripherally related to 



