734 NITROGEN METABOLISM AND GROWTH 9 



logical inquiry are the echinoderm, the amphibian and the chick. They serve 

 different purposes. We are not reluctant to accept as developmental generalities the 

 findings for each, yet we do appear to be somewhat hesitant in applying the 

 principles of physiological genetics from Drosophila or microorganisms to our 

 problem. It would seem unwise to neglect these basic findings. 



The direct involvement of genetic control of metabolism in development of 

 higher organisms is lacking, and probably must remain so. The complexity and 

 interrelationships of metabolic cycles is sufficient indication of the virtual impos- 

 sibility of genetic interference without resulting in death. It is for this reason that 

 the mutations which have shed most light on development have been lethal ones, 

 and it is likely the same background of reasoning which has given rise to the 

 controversy over pleiotropy. Since qualitative genie effects are more easily noticed 

 and more easily measured than quantitative effects which would be related to 

 growth, it is these which have been stressed by developmental geneticists. Gruene- 

 berg (1951) describes the multifactorial inheritance of absence of the third molar 

 in mice as a case of maternal potentiation of the absence of the smallest teeth 

 in a series. He refers to this (1952) as quasicontinuous variation and conceives of 

 the threshold involved not as a quantity of dental lamina but of factors, related 

 to maternal physiology, which may not be the same from litter to litter. Because, 

 however, of the extreme likelihood that the same kind of mechanism is involved 

 in continued synthesis of protein as in the synthesis of a new type of protein, the 

 principles established in the study of differentiation must certainly also apply 

 to growth. 



The pertinent details of metabolic studies of Neurospora have been adequately 

 and lucidly reviewed by Wagner and Mitchell (1955); they have incorporated 

 significant studies on other forms used in genetic metabolic studies, e.g., Drosophila 

 and man. The studies of parthenogenetically stimulated animals are not without 

 interest in the sense that successful development of haploid embryos implies either 

 that the essential developmental processes are not under genetic control or that, 

 if they are, the genes responsible are almost entirely homozygous. The latter is 

 obviously not a remote possibility, particularly in light of the fact that literally 

 hundreds of examples of gene-controlled metabolic processes have been studied. 

 The interference with gastrulation by hybridization in Amphibia is consonant 

 with such a notion (Moore, J., 1941, '46, '47; Sze, 1953; Gregg, 1948). The 

 experiments of Briggs and King on nuclear transfer point to a nuclear differen- 

 tiation, suggesting on a genetic hypothesis, irrevocable nuclear (genie) inacti- 

 vation with the course of time in development. Such a view is supported also by 

 Brachet (1952) and by Waddington (1956). Moore (1950) feels that her results 

 with androgenetic frog hybrids "suggest that the abnormal development and 

 death of haploid amphibian embryos is due to both haploidy per se and to 

 unmasked lethal and semilethal genes." Her review of the time of death of 

 haploid amphibian embryos of different species raises interesting questions about 

 the variability which presumably exists with regard to the nature of genetic 

 control of similar processes in related animals. 



That the genome does exert control over development is established by studies 

 of Drosophila (Hadorn, 1955; Poulson, 1940, 1945) of the mouse (reviews by 



