740 



NITROGEN METABOLISM AND GROWTH 



A summary of partition of nitrogen in the several species studied is shown in 

 Fig. 5. From this it is clear that from these data no pattern of biochemical 

 evolution is apparent. Each species excretes some of each of the three major 

 excretory compovmds. The enzymatic mechanism for uric acid synthesis, urea 

 synthesis and ammonia production is therefore present. Only in the chick is the 

 predominant excretory product uric acid, yet the egg of each species studied, 

 except the viviparous garter snake, is cleidoic. No excreta are lost through the 

 shell except gaseous ammonia, and this in relatively small amounts at the be- 

 ginning of development. 



The excretory "efficiency" of the several species (Table i) is of interest in terms 

 of its variability. There is apparently no uniformly operative mechanism for the 

 prevention of protein combustion among these cleidoic eggs. The low figure for 

 the garter snake may be partially explained by loss of excreta through the 

 placenta and by the transmission of amino acids to the embryo (Clark, Florio and 

 Hurowitz, 1955). Such an explanation cannot, however, suffice for the alligator, 

 described in detail by Clark et al. (1957). 



One of the interesting features of the process of excretion by the reptiles and 

 the chick is the mechanism responsible. The existence of urease activity in the 

 black snake embryo concomitant with a rise in uric acid synthesis and decline 

 in stored urea led to the further examination of the activity of this enzyme. 

 Experiments were carried out on the garter snake (Clark and Sisken, 1956) to 

 test the reality of its activity. Injection of ^^N urea into the peritoneal cavity and 

 yolk sac and examination of uric acid for ^^N were made. These indicated that 

 22-56% of the ^^N was recoverable in the adult uric acid after 48 h. These results 

 point rather clearly to an active, physiological role of urease in excretion of the 



90 



70 



^50 



0-30 



10 



Ammonia | | 



Urea | | 



Uric acid 



ALLIGATOR 



TURTLE 



SNAKE 



CHICK 



Fig. 5. Partition of excreted nitrogen among urea, ammonia and uric acid, shown as 

 percentage of total nitrogen excreted, by embryos of alligator, snapping turtle, black snake 

 and chick to hatching. There appears to be no systematic sequence. All eggs are cleidoic. 



