Chapter 9 



NITROGEN METABOLISM AND GROWTH 



Hugh Clark 



I. INTRODUCTION 



The adult metazoan organism is an integrated composite of a great variety of 

 cell types, similar in some respects but different in morphological and physio- 

 logical qualities which identify them. One feature of individual distinctions is the 

 complement of peculiar proteins, both enzymatic and nonenzymatic, which 

 contribute in large measure to the identifiable individuality of the cell. 



It is our purpose to examine the available data which describe, with respect to proteins, 

 (/) the nature of the egg and sperm, {2) the pattern of acquiring diverse structural and 

 enzymatic peculiarities, (j) where evidence is at hand, the mechanisms involved, including 

 particularly energy requirements, and {4) a statement of residual problems. It should be 

 unnecessary to add that details of all aspects of protein metabolism in all stages of development 

 cannot be pursued with equal fidelity. 



The author has included comprehensive and review papers in areas which have 

 been particularly active and has cited original contributions which are germane 

 to the main theme of the present review. Limitation of space forbids citation of 

 all pertinent material and perhaps has resulted in serious omissions. It is hoped, 

 however, that the most significant contributions and the most effective techniques 

 of study have not been overlooked. For purposes of orientation, two aspects of 

 the problem will be defined at some length, (A) growth and development, and (B) 

 metabolism of nitrogen compounds. 



{a) Growth and development 



Phenomena of growth have appealed to the biologist since, and perhaps before, the 

 dawn of recorded observations. The observations are varied and, certainly in many details, 

 conflicting. The reasons are summarized by Weiss (1949): "Growth is not a simple and 

 unitary phenomenon. Growth is a word, a term, a notion, covering a variety of diverse 

 and complex phenomena. It is not even a scientific term with defined and constant 

 meaning, but a popular label that varies with the accidental traditions, predilections and 

 purposes of the individual or school using it. It has come to connote all and any of these: 

 reproduction, increase in dimensions, linear increase, gain in weight, gain in organic mass, 

 cell multiplication, mitosis, cell migration, protein synthesis and perhaps more." Earlier 

 in the same paper Weiss proposes to "define growth in a more restricted sense, as the 

 increase in that part of a molecular population of an organic system which is synthesized 

 in that system". But even this refinement of concept does not eliminate the phenomena of 



