Energy Exchange and Enzyme Development During Embryogenesis 537 



experiments how much protein was used 

 before the animals were capable of feeding, 

 or the extent to which the animals were 

 forced by starvation to call upon their tissue 

 reserves. It seems reasonable to conclude, 

 however, that the degree of inanition was 

 not very great at the time his observations 

 were terminated, for the respiratory rate 

 had not yet begun to decline as it does dur- 

 ing starvation. 



The results of chemical analyses of the 

 changes in total nitrogen during develop- 

 ment apparently do not support the view 

 that protein serves as an energy source. In 

 contrast with the earlier findings which 



but the changes observed were very slight 

 and did not exceed in magnitude the vari- 

 ation between different samples of eggs or 

 embryos.* 



The amphibian egg consists of a large 

 amount of protein in relation to the total 

 dry weight; Needham ('31, p. 1105) gives a 

 figure of 61 per cent. Most of this protein is 

 stored as yolk and serves as a reserve on 

 which the embryo draws for the elaboration 

 of new materials during growth and differen- 

 tiation. In view of the great quantity of 

 protein in the egg at the beginning, any de- 

 crease, as by combustion, is bound to be 

 relatively small. Hence what appears to be 



Table 20. Total Nitrogen in Amphibian >Embryos\ 



SPECIES MG. N INITIAL HG. N FINAL % DIFFERENCE 



Amblvstoma punctatum 430 430 



Amblystoma tigrinum 290 280 —3.5 



Triturus torosus 260 230 —12 



Rana pipiens 160 190 -f 1 3 



t Data from Wills ('36). In computing the average figures shown above, the first determinations at the 

 beginning of development and the last determinations before the feeding stages were used. The number of 

 determinations represented in the averages are as follows: for Amblystoma punctatum and Triturus torosus, the 

 first five and the last five; for ^. tigrinum, the first two and the last two; for Rana pipiens, the first nine and 

 the last nine. 



showed some loss of protein by combustion. 

 Wills ('36) reported that total nitrogen was 

 essentially constant throughout the pre- 

 feeding period of development in four species 

 of amphibia: Amblystoma punctatum, A. 

 tigrinum, Triturus torosus, and Rana pipiens. 

 A portion of Wills' analyses have been ab- 

 stracted and are shown in Table 20. Wills 

 included the egg jelly in the material an- 

 alyzed, but he claimed that its presence did 

 not materially affect the quantity of nitrogen 

 measured. This view is contested by Gregg 

 and Ballentine ('46), who found that jelly 

 nitrogen averages 16 micrograms per embryo, 

 that is about 10 per cent of the total nitrogen 

 in egg plus jelly, and that the vitelline mem- 

 brane contains about two micrograms. They 

 did, however, confirm Wills, as did also 

 L^vtrup ('53a), by stating that total nitrogen 

 remains nearly constant throughout develop- 

 ment. A minor decrease noted at stage 16+ 

 (Shumway) was attributed to loss of the 

 vitelline membrane and the fluid contained 

 in the vitelline space. Some change in the 

 distribution of nitrogen in various constitu- 

 ents of the embryo was found, indicating that 

 protein metabolism was going on even 

 though it did not eventuate in combustion, 



constancy of protein may simply mean that 

 changes in total nitrogen are obscured by 

 variability in the samples of eggs and em- 

 bryos used for analysis. That this factor may 

 not have been adequately taken into account 

 is illustrated both in the data of Gregg and 

 Ballentine and in those of Wills. Gregg and 

 Ballentine obtained an average figure of 

 162 Mg. total nitrogen per egg at the be- 

 ginning of development from a series of nine 

 samples which ranged from 141 to 202 fig. 

 The standard deviation for their array of 

 figures is 17.2 Mg., or 10.6 per cent and this 

 corresponds to 107 fig. of protein. For the 

 combustion of this amount 102 fA. of oxygen 

 would be required. This could mean, using 

 Atlas' ('38) figure of 250 fil. for the total 

 oxygen consumption of one Rana pipiens 

 embryo throughout development, that as 



* They found also that the quantity of ultracen- 

 trifugable nitrogen, presumably representing gran- 

 ular material, is constant throughout development. 

 This is interesting in view of the fact that much of 

 the RNA of the cell is located in such granules and 

 that Bodenstein and Kondritzer ('48), Kutsky 

 ('50), and Krugelis, Nicholas, and Vosgian ('52) 

 showed that RNA increases between ten- and 

 twenty-fold. 



