536 Energy Exchange and Enzyme Development During Embryogenesis 



gins at gastrulation. The values obtained 

 were 0.66 for early cleavage stages, 0.70 for 

 advanced blastulas, and 1.03 for gastrulas. 

 Dtiring subsequent development, the respir- 

 atory quotient fell slightly. A series of re- 

 spiratory quotient values for R. pipiens em- 

 bryos, during and after gastrulation, are 

 shown in Table 18 (Boell, unpublished). 

 The data in the table confirm the findings 

 of Brachet for respiratory quotients of ap- 

 proximately imity during gastrulation. Un- 

 fortunately, determinations were not made 

 during cleavage stages. However, additional 

 evidence that low values are characteristic 



inally present in the egg. That protein is 

 burned during development is indicated by 

 the fact that ammonia and urea are excreted 

 by the embryo (Brachet, '39, see columns 1 

 and 2 of Table 19). Boell, Needham, and 

 Rogers ('39) reported that ammonia was 

 produced anaerobically by explants of Rana 

 temporaria gastrulas, but this result may be 

 in error in view of Gregg and Ornstein's 

 ('52) inability to find ammonia production 

 by explanted tissue of R. pipiens. Column 

 7 of Table 19 suggests that a not inconsidera- 

 ble portion of the total oxygen consumption 

 can be accounted for as protein combustion. 



Table 19. Utilization of Protein as Energy Source by Rana fusca^ 



NITROGEN EXCRETED BY 

 100 EMBRYOS PER DAY 



PROTEIN O2 NEEDED TO Oo UPTAKE % O2 UPTAKE 

 EQUIV. TO BURN PROTEIN OBSERVED f AS PROTEIN 



* Figures in columns 1, 2, 3, and 4 represent ^g-, in columns 5 and 6, yul- 



t Data for oxygen consumption in column 6 from Brachet ('34); for columns 1 and 2, from Brachet ('39). 



of the period before gastrulation may be 

 seen in the observation that explants from 

 the blastula of A. mexicanum have a re- 

 spiratory quotient of 0.75 (Boell, Koch, and 

 Needham, '39). Barth ('46) has also obtained 

 high vakies for the respiratory quotient dur- 

 ing gastrvilation and later development, al- 

 though the average is somewhat lower than 

 in the experiments already mentioned. This 

 may possibly be due to the fact that oxygen 

 consumption and carbon dioxide output were 

 not determined on the same sample of eggs. 

 Barth measured the respiratory quotients of 

 cleavage stages but found that they were 

 only slightly lower than during gastrulation. 

 However, Barth apparently feels that the 

 absolute value of the respiratory quotient 

 for early cleavage must be regarded as un- 

 known from his work, for his figures varied 

 from 0.7 to 1.05, depending somewhat on the 

 method used. 



The question of protein as an energy 

 source is highly confused. Most of the data 

 summarized by Needham ('31) showed com- 

 bustion of protein before hatching and some- 

 what enhanced utilization afterward. There 

 was, however, little agreement as to the 

 amount lost in relation to the amount orig- 



and that relatively more energy may be de- 

 rived from protein combustion during early 

 development than later. These calculations, 

 taken with the observations on carbohydrate 

 catabolism, sviggest that protein utilization 

 precedes that of carbohydrate, and it prob- 

 ably continues throughout all of develop- 

 ment. 



The question of energy source utilization 

 in A. mexicanum has been investigated by 

 L0vtrup ('53a) in a stvidy combining re- 

 spiratory and reduced weight measurements. 

 His calculations apparently show that car- 

 bohydrate is used exclusively as an energy 

 source during cleavage stages. But reduced 

 weight data are of limited utility in estab- 

 lishiHg this convincingly, and Lc*vtrvip states 

 that "the chemical analyses must of course 

 be decisive on this point." He has also cal- 

 culated that protein may serve as an energy 

 source and that the amount of protein so 

 used corresponds to 16 per cent of that in- 

 itially in the egg. Since his observations were 

 continued into the feeding period, Ldvtrup 

 has concluded that some of the protein lost, 

 perhaps a major portion, might have been 

 spared had his animals been permitted to 

 feed. It is not possible to determine in his 



