74.2 NITROGEN METABOLISM AND GROWTH 9 



integration of developmental phenomena in an organism and the delicate balance 

 between the factors of growth, differentiation and maintenance have circum- 

 scribed the sphere of activity of developmental biologists. Organizational capa- 

 cities and their interrelations have been explored at the levels of cells, tissues 

 and organs. One can speak with reasonable assurance about the developmental 

 prospects of areas of a developing embryo in terms of growth, differentiation, 

 capacity for regeneration, metabolic requirements and the conditions for their 

 realization (at least, in terms of a variety of conditions which prevent their 

 realization) . 



It becomes apparent, however, that molecular details of development are 

 essential to a complete understanding of the higher categories of organismic 

 epigenesis. The need for such details has for a long time been recognized. Other 

 groups of biologists have demonstrated similar interests by the continuous attention 

 through the years to metabolic aspects of cellular behavior and the correlation 

 of these with cytomorphological entities; by attention to the relations between 

 metabolic activities and genie behavior on the one hand and phenotype on the 

 other; by attention to analysis and synthesis of proteins; by attention to the 

 ultramicroscopic structure particularly of proteins and the alteration of cellular 

 behavior by delicate physical techniques. Confluence of the several streams of 

 activity on developmental questions has enabled the developmental biologist to 

 proceed intensively with the analysis of some molecular aspects of development, 

 particularly with regard to protein. Hence work has been accomplished along 

 the following lines: (j) quantity of total protein and non-protein nitrogen of 

 entire embryos and their parts, (2) use of total protein or specific proteins as an 

 index of growth, (j) constitution of the non-protein nitrogen pool as an index of 

 protein metabolism; {4) identification of proteins by virtue of specific immuno- 

 logical reactions; (5) the physical shape and size of protein molecules in relation 

 to morphogenetic phenomena; (6) the permanence of the original store of protein 

 and its use as an energy source, as determined by analysis both of residual protein 

 and excreted nitrogen; (7) cytological mechanisms responsible for synthesis of 

 proteins; (8) enzymatic studies concerned both with rate of accumulation and 

 role in metabolic activities; (9) the behavior of proteins antigenically, either 

 directly as in the events of fertihzation or indirectly as may be involved in growth 

 or its inhibition. 



The investigations outlined above are carried out in a variety of animals; 

 certain general similarities between developmental patterns in the diverse objects 

 of study can acquire specific meaning only in light of events at the molecular 

 level. Synthetic processes and the cytological mechanisms responsible for them 

 may well be similar from organism to organism; the identification of the simi- 

 larities of processes may clarify the major sources of difference between, say, 

 alecithal and telolecithal eggs. 



The most pressing residual questions for the developmental biologist are the 

 degree of genetic control of development and the intracellular mechanisms 

 responsible for it; the physiological and morphological characteristics of differen- 

 tiation at t^ie cellular level; the nature of the molecular population of individual 

 cells as an index both of differentiation and its manner of influencing other cells. 



