736 NITROGEN METABOLISM AND GROWTH 9 



accounting for the synthesis of Hving structures with a fairly modest expenditure 

 of food." 



Needham (1931, 1942), Tyler (1942) and Brachet (1950) have reviewed 

 evidence and offered interpretations on theoretical grounds of the relative 

 apportionment of energy among the several aspects of development (growth, 

 differentiation, mechanical movements, organization and maintenance). Our 

 earlier decision (pp. 713—715) that such segregation of processes is unprofitable, 

 if not impossible, focuses attention on the question of energy sources for the 

 manifold but integrated operations of development, and in particular the extent 

 of protein utilization for such purposes. 



Among the "provisional generalizations for chemical embryology" Needham 

 (1931, III, pp. 1656-1657) includes "a succession of sources of energy, carbo- 

 hydrate preceding protein and protein preceding fat", and "cleidoic eggs possess 

 metabolic peculiarities : (a) suppression of protein catabolism, {b) an emphasis 

 on fat catabolism, and (c) election of uric acid as the main end-product of protein 

 degradation". 



Needham's monumental compilation of chemical data relating to development 

 has been a fountain head of information for embryologists of the past generation, 

 and his conclusions are widely quoted. If we accept the principle that nitrogenous 

 waste products are evidence of catabolism of protein, serving primarily as an 

 energy source, we must reexamine some of Needham's conclusions more critically. 



b. Succession of energy sources. A sufficient body of information about development 

 of both vertebrate and invertebrate animals has accumulated since 1931 that 

 Needham's carbohydrate-protein-fat sequence of energy sources has lost its 

 significance through contradictions. Hollett and Hayes (1946) find that 44.5% 

 of the initially stored protein is used for energy production, accounting for 40% 

 of the energy required up to the time of yolk sac absorption of the Atlantic 

 salmon, Salmo salar. The sequence in this species appears to be fat-protein-fat- 

 protein, and carbohydrate plays only a very minor role in energy supply. Smith 

 (1946), studying the trout likewise finds a continuous increase in nitrogen 

 excretion throughout development. In 1952, Smith reports that carbohydrate 

 combustion is confined to (j) a period immediately after gastrulation, (2) during 

 hatching and (j) onset of starvation. He cannot substantiate peaks of carbohydrate, 

 protein and fat combustion, but sees some evidence for an initial peak of protein 

 + phosphatide fat and a later one of glyceride fat. He makes the interesting 

 observation that combustion of phosphatide fats is conspicuous during yolk sac 

 absorption, "and may be correlated with consumption of protein as an energy 

 source." Gregg and Ballentine (1946) were able to find little evidence for protein 

 breakdown early in frog development, though Brachet (1939) had found evidence 

 for protein utilization in pregastrula embryos. Boell, Needham and Rogers (1939) 

 observed that ammonia production of dorsal lip explants was three times as great 

 as that of ventral ectoderm under similar conditions. Gregg and Ornstein (1952), 

 however, find only traces of ammonia excreted by either type of explant and 

 attribute the results of Boell et al. to liberation of ammonia from cytolizing cells. 

 Carbohydrate utilization prior to gastrulation has not been unequivocally 



