73^ NITROGEN METABOLISM AND GROWTH g 



Careful study of the data compiled by Needham (1931) indicates that the 

 earliest period of development was uniformly neglected. Although the data 

 presented in Fig. 4 do not deny the possibility of carbohydrate combustion during 

 the earliest stages of development in the chick and various reptiles, they do focus 

 attention on the hitherto neglected importance of protein as an energy source. 

 Moreover, the peak of protein utilization in the chick at 8-g days, described by 

 Needham and related to protein catabolic events, is seen to disappear. With the 

 curves of protein combustion presented in Fig. 4, one cannot be impressed by the 

 8-9 day peak. A word of possible explanation is offered. Fiske and Boyden (1926), 

 upon whose data Needham relies heavily, analyzed only the allantoic contents. 

 Immediately after 7 days when uric acid synthesis proceeds rapidly, the evidence 

 for burned protein would loom large. In actuality, both urea and uric acid are 

 produced from the beginning of development; the findings with respect to 

 ammonia are less certain for the reason that the amount found in the unincubated 

 egg is already significant, and it is difficult, if not impossible, to distinguish 

 between stored ammonium salts and newly created ammonia. No mechanism 

 for the transfer of proteins to the embryo has been described for the chick, such 

 as the possible "carrier" role of phosphatides as described by Smith (1952). 

 Such a mechanism, however, would appear to warrant investigation. 



c. Nitrogen metabolism in cleidoic eggs. The term cleidoic (Gr. I enclose) describes 

 the essentially "closed box" eggs of terrestrial, non-viviparous vertebrates. The 

 structure implies that the mother must include within the shell all non-gaseous 

 materials necessary for the formation of an embryo. A further implication is that 

 excretory waste must also be confined within the shell until hatching. In a careful 

 study of the excreta of the chick embryo Fiske and Boyden (1926) point out that 

 excretion of uric acid is virtually a necessity, since "a substance as soluble and 

 diffusible as urea could not possibly replace it as an end-product when the 

 organism and its excretions are confined to a closed system, the walls of which 

 are only permeable to matter in the gaseous state". Needham (1931) continuing 

 in the same line of thought calculates that, if all the stored nitrogenous waste 

 existed in the form of urea, the concentration of urea would be approximately 

 165 mg%, which "is undoubtedly of the pathological order of magnitude, and if the 

 avian embryo had to suffer from a constant headache and other symptoms before 

 hatching, natural selection would hardly have preserved it for our entertainment". 

 The conclusions just stated, as well as those in the foregoing section, based largely 

 on chick analyses were extrapolated to describe the situation for reptiles as well. 

 The unusual opportunity presented itself to make a study of nitrogen partition 

 in embryos of the black snake. The findings on the black snake were sufficiently 

 different from the "sauropsid pattern" commonly described, that I was led into 

 similar studies of other reptiles and a reexamination of the chick. Since the 

 results fall into a pattern whose meaning is substantially different from that 

 heretofore accepted, they will be described at some length^ 



^ The work of the author herein described was supported in part by a grant (G 3827) from 

 the Division of Research Grants, National Institute of Health. 



