I GROWTH AND DEVELOPMENT 715 



events at the cellular level since it is here that metabolism of nitnjgen compounds 

 must be observed. Applications of nitrogen metabolism to higher levels of organi- 

 zation are not wanting, however, and these will be reported in their proper place. 

 Ordinarily such application is invoked in terms of energy supply for develop- 

 mental work. Proteins are involved as an energy source in two ways: first, by 

 hydrolysis, deamination of constituent amino acids and oxidation of the carbo- 

 hydrate skeleton, and secondly as agents in formation and use of high energy 

 phosphate bonds (Barth and Barth, 1954). With regard to the latter possibility, 

 however, Lovtrup (1953) by rather conclusive calculations demonstrates that 

 amphibian yolk phosphoproteins could supply not more than 0.15% of the 

 required energy. 



Of paramount importance in the concept of developmental phenomena is 

 the organismic nature of the embryo. The processes concerned with growth, 

 differentiation and maintenance are indissolubly intertwined. These, in turn, 

 cannot be isolated from their environment; no embryo, even cleidoic, constitutes 

 a closed thermodynamic system. Each has a direct, continuing and varying 

 relationship with its surroundings in the present, and each has, through the germ 

 line an extension into the past. Recent serological studies, which identify antigens 

 of the egg with those of the adult and of the developing embryo, assign to the 

 latter premise a tangible significance. 



(b) Metabolism of nitrogen compounds 



Nitrogen metabolism must include, because it constitutes prima facie evidence, 

 excretion of nitrogenous waste. This implies a degradation of proteins, deami- 

 nation and a failure of transamination or transamidation into protoplasmic 

 nitrogen compounds of continuing service to the organism. As an immediate 

 consequence ammonium salts are formed; these may be eliminated without 

 change; free ammonia may be evolved, or, if the enzymatic machinery and the 

 conditions necessary for its operation are present, the ammonium compounds 

 may be diverted into more innocuous channels such as urea or uric acid formation 

 (or in smaller quantities to other compounds). Nitrogen excretion may be 

 construed as a measure of utilization of nitrogen compounds for purposes of 

 providing energy for maintenance or development; it must, however, be inversely 

 correlated with direct transformation of stored nitrogen compounds into the 

 protoplasmic substance of the developing organism. 



In addition, however^ nitrogen metabolism must also include such anabolic 

 processes as {a) synthesis of yolk, thus establishing a nutritional link between 

 parent and offspring; (b) transfer of yolk protein to the embryo; (c) synthesis of 

 structural and enzymatic proteins of the embryo, with perhaps, a corollary of the 

 acquisition of antigenic capacities; (d) duplication of both nuclear and cyto- 

 plasmic components ; (e) factors related to the definitive structure of cell mem- 

 branes and (/) the functional interdependence of nucleus and cytoplasm. 



Prior to 1931, as Needham's (1931) magnificent compilation and synthesis 

 clearly shows, chemical embryology leaned heavily on routine analyses for 

 information regarding protein metabolism; such analyses, coupled with respira- 

 tory data constitute much of the basis for Needham's conclusions. Up to this time. 



Literature p. 744 



