II ORGANOGENESIS 737 



demonstrated in the frog, though Lovtrup (1953) after correlation of reduced 

 weight, oxygen consumption and chemical analyses points out that there is a 

 sequence of carbohydrate-protein-fat in A. mexicanum. Relative amounts used 

 by the embryo during the period of observation were carbohydrate: 125 fxg; 

 protein: 196 [jig; and fat 204 [xg. The protein figure is uncertain; it "might have 

 been lower if feeding had been allowed". Lovtrup points out that yolk phospho- 

 protein is not used prior to the tail bud stage and makes the curious observation 

 that "yolk utilization is not directly related to protein combustion". Boell (1955) 

 makes a more extended analysis of energy sources in amphibia and cites variability 

 in size, hence protein content, of eggs as a possible explanation for conflicting 

 results. If doubt exists with regard to the relative importance of protein 

 as an energy source in early amphibian ontogenesis, none is apparently warranted 

 with respect to alteration in protein structure during this period. Kutsky et al. 

 (1953) have measured in amphibia quantitative changes in amino acids and find 

 that arginine, leucine, lysine, methionine, threonine and tyrosine seem to increase 

 slightly with minor fluctuations, while aspartic acid, glycine, histidine and threo- 

 nine decrease to midblastula and increase through gastrulation to a maximum at 

 midneurulation. Most clear-cut of their observations is the behavior of glutamic 

 acid, which increases through cleavage, drops sharply at midgastrulation, in- 

 creases in late gastrulation and drops again until neurulation. They feel that the 

 increase during cleavage "may be linked to the subsequent formation of specific 

 proteins of the induction or neuralization process". 



Allusion has previously been made (pp. 723) to studies on protein metabolism 

 of sea urchin embryos. The evidence appears to be consistent that continuous 

 utilization of protein occurs during early development, and (Hayes, 1938) that 

 protein combustion is supplemented by fat combustion. Indications of carbo- 

 hydrate utilization during this period are based heavily on measurements of 

 respiratory quotient. Such measurements are of some assistance in making a 

 decision of substrate utilization but are not completely reliable. Compounding 

 the difficulties attending the real meaning of respiratory quotients is the added 

 confusion of conflicting data (see Boell, 1955). 



Needham's postulates (193 1), though stated in general terms, are based heavily 

 on data pertaining to chick development. The doubts with regard to general 

 applicability of the carbohydrate-protein-fat succession are now extended to the 

 chick and certain reptiles. In the black snake (Clark, 1953b) the R.Q,. during 

 the first week after t^g laying is approximately 0.8, but drops thereafter and 

 remains consistently near 0.53 for the remainder of the incubation period of 

 10 weeks. However, the rate of nitrogen excretion is highest during the earlier 

 period; it would appear therefore, that, although carbohydrate may be burned 

 during this period, certainly also protein contributes greatly to combustible food 

 sources. The rate of protein destruction is 25 times greater at the beginning of 

 development than at the end, with the sharpest drop coming during the second 

 week. A similar phenomenon has been described for the garter snake, a placental 

 species (Clark and Sisken, 1956), and for the alligator (Clark et al., 1957)- 

 The same kind of observation has also been made on the chick and the snapping 

 turtle. These data are summarized in Fig. 4. 



Literature p. 744 



