Energy Exchange and Enzyme Development During Embryogenesis 535 



the question of energy sources. It is probably 

 derived from carbohydrate, for Lindberg 

 ('43) was able to account for it quantita- 

 tively by the amount of glycogen lost at 

 fertilization. Moreover, Rothschild ('39) was 

 able to inhibit the production of acid in 

 cytolyzing eggs with phlorizin, but it should 

 be noted that the production of acid vmder 

 these circumstances may not be the same as 

 in intact eggs. The acid is not lactic acid 

 (Runnstrom, '33; Rothschild, '39), and there 

 is little likelihood that it is fatty acid, as 

 sviggested by Hayes ('38). Lindberg believes 

 that the acid is not a single substance, but 

 a mixture of intermediates of carbohydrate 

 breakdown. 



More recent studies by Ohman ('40) in- 

 dicate that in Paracentrotus lividus the 

 respiratory quotient is 0.73 after fertiliza- 

 tion; it rises during subseqvient development 

 to a value of 0.85. Laser and Rothschild ('39) 

 also noted that the respiratory quotient was 

 low immediately after fertilization. In 

 Arbacia punctulata, the respiratory quotient 

 rises after fertilization, but the initial value 

 (0.85) is not as low as that reported by 

 Ohman nor is the increase so marked (Hut- 

 chens, Keltch, Krahl, and Clowes, '42). It 

 may be significant that the rise in respiratory 

 quotient is temporally correlated with the 

 development of lithium sensitivity, for Lin- 

 dahl ('39) has suggested that this fraction 

 of respiration involves carbohydrate catabo- 

 lism. The low initial respiratory quotient is 

 consonant with the finding by Hutchens, 

 Keltch, Krahl, and Clowes ('42) that, during 

 the first few hours after fertilization, the 

 small quantities of glucose lost from the 

 egg of Arbacia cannot account for the oxygen 

 uptake. It also agrees with Lindberg's ('43) 

 observation that glycogen loss, after an in- 

 itial rapid breakdown associated with acid 

 formation, proceeds at a low and fairly con- 

 stant rate. It is also in harmony with the 

 report by Hayes ('38) that fat is lost from 

 the egg throughout development (45 hours) 

 but that the rate of disappearance is most 

 rapid during the three or four hours im- 

 mediately after fertilization. 



Gustafson and Hasselberg ('51) have re- 

 ported that Kjeldahl nitrogen is constant 

 from fertilization to the pluteus stage in 

 Paracentrotus lividus (see the discussion on 

 p. 520). Nevertheless, some protein must be 

 burned, for Hutchens, Keltch, Krahl, and 

 Clowes ('42) have shown that protein com- 

 bustion leading to ammonia as the end prod- 

 uct can account for a portion of the oxygen 

 consumption after fertilization. These col- 



lective findings thus suggest that fat is prob- 

 ably the major energy source for a short 

 time after fertilization and that during the 

 period of exponential increase in respira- 

 tion metabolism is mixed but with carbo- 

 hydrate becoming increasingly important 

 as development proceeds. It is of interest 

 in passing to note that almost 90 per cent of 

 the respiration of sea urchin spermatozoa 

 can be accounted for by oxidation of phos- 

 pholipid (Rothschild and Cleland, '52). 



The Amphibian Embryo. The sequence of 

 energy sources in Amphibia was recognized 

 by Needham ('42) as being different from 

 that in the chick. Carbohydrate loss during 

 early stages has not been demonstrated, but 

 that it begins at gastrvdation has been shown 



Table 18. Respiratory Quotients of Rana 

 pipiens Embryos 



* The first method of Dickens and Simer was used 

 in making these determinations (Dixon, '51). 



by histochemical tests (Woerdeman, '33; 

 Raven, '35) and chemical analyses (Brachet 

 and Needham, '35; Heatley and Lindahl, 

 '37; Jaeger, '45; Gregg, '48). The claim of 

 Gregg and Pomerat ('42) that glycogen dis- 

 appears during cleavage has been withdrawn 

 by Gregg. Loss of glycogen determined by 

 chemical analysis, during development of 

 Amblystoma mexicanum has been reported 

 by Lcivtrup ('53a), but his analyses were 

 confined to the beginning and end of devel- 

 opment and hence do not show when con- 

 sumption begins. 



That disappearance of glycogen is due to 

 combustion rather than storage in the form 

 of some other carbohydrate has been sug- 

 gested by Gregg, since he found that the 

 total free carbohydrate remains constant at 

 a very low level until late in development. 

 This seems reasonable, but it does not elim- 

 inate the possibility that glycogen may be 

 synthesized into some non-carbohydrate 

 product. 



Brachet's ('34) study of the respiratory 

 quotient dviring development in Rana fusca 

 is in harmony with the chemical analyses 

 in showing that carbohydrate utilization be- 



