730 NITROGEN METABOLISM AND GROWTH 9 



even as in the adult ovary; and further, the phenomena of growth, differentiation 

 and maintenance, are the result of such local limitations. 



Such a concept, perhaps unhappily, leaves essentially the same developmental 

 problems which have tormented the ininds of embryologists for three quarters of a 

 century. Recent studies with immunological techniques and paper chromatography 

 enable us to chart more successfully the milestones of progress by the embryo, 

 and to a limited extent, the methods used by the embryo. It is in the latter connec- 

 tion that primary interest lies with respect to protein and nitrogen metabolism. 

 Nitrogen metabolism may l)e related in two ways: (j) in its relation to protein 

 accumulation, both replication and differentiation, and (2) nitrogen compounds 

 as energy sources for the reactions involved in (j). The enzymatic maturation 

 of the embryo then has both direct and indirect significance. 



Enzymes are among the easiest proteins to characterize, because of specific 

 relation to substrate. Moog (1952) has summarized the extensive literature on 

 enzymes in the developing chick. She announces three major conclusions with 

 supporting evidence: (j) young embryonic tissue is enzymatically complex, 

 (2) enzymogenesis proceeds at different rates for different enzymes, and (j) enzyme 

 accumulation is an aspect of functional differentiation. She brings to bear on 

 the first point evidence for the presence, during the first day, of amylase, lipase, 

 protease, cytochrome oxidase, alkaline phosphatase and acid phosphatase, and 

 during the second day, dipeptidase and aminopeptidase. In support of the second 

 point, she has collected information from various sources about the rate of 

 accumulation of the enzymes mentioned, and in addition, that of arginase. Of all 

 enzymes studied, only the last shows a continuous trend (downward) and she 

 feels that fluctuation in activity is a characteristic of each in early development. 

 The last conclusion is supported (j) by calculating different rates of accumulation 

 of the same enzymes in different tissues, e.g., apyrase in brain, liver and muscle 

 and (2) by relation of enzyme concentration to onset of functional activity in 

 nervous tissue, muscle, kidney, intestinal epithelium and liver. 



Boell (1955) extends the review to embryos other than the chick. He cautions 

 against the conclusion of causal relationship between enzyme development and 

 the process with which it is associated. 



2. Growth Regulation 



a. Differential growth. A large literature exists with regard to differential growth, 

 for which Huxley (1932) offered, on the basis of a wide variety of examples the 

 formulation of the heterogonic growth process as j' = bx'^, in which j)^ is a meas- 

 urement of one dimension of growth, x is the corresponding measurement of 

 the entire animal (or smaller unit of which x is only a part, e.g. face and head), 

 and b and k are constants. It is apparent that relationship between rates of 

 change of two measurements may be expressed by the relationship between b 

 and k. Needham (1934) substantiated the usefulness of the concept for "chemical 

 growth". Brody (1945) exploited a modification and refinement of this concept 

 calculated to give, for any period of development, the instantaneous growth rate 

 by use of a plot of the logarithm of the growth dimension against time. This 



