200 



Comparative Animal Physiology 



Lizards and Snakes. Water conservation is important for lizards and snakes; 

 also their eggs must retain embryonic nitrogenous excretion until they are 

 hatched. Hence they excrete most of their nitrogen as the relatively insoluble, 

 innocuous uric acid (Table 30). In Phrynosoma, the horned toad, the urine 

 is a solid ball of uric acid. 



Birds. Birds con\'ert most of the ammonia from protein to uric acid (Table 

 30). Their urine is a semisoHd mass of uric acid crystals. In the chick embryo 

 the first product is ammonia, which declines rapidly in amount after the fourth 

 day of incubation. Urea increases to a maximum at the ninth day and then 

 declines; the enzyme arginase falls to a minimum at about the twelfth day. 

 The excretion of uric acid on a dry weight percentage basis is maximal at the 

 eleventh day, and uric acid is the principal adult product (Fig. 35). '""'*• *"'• "^'^ 

 If embryonic tissues were separated from extraembryonic membranes the 

 absolute amounts excreted by the embryo might differ from those given in 

 Figure 35, but the general sequence is significant. 



Uric acid is found in body fluids of animals in which it is not the principal 



Fig. 35. Excretion in chick embryos. From Needham.' 



excretory product. ^" For example, in annelids the uric acid concentration in 

 coelomic fluid is 0.73 mg./lOO ml. in Chaetoptenis "^ and 0.1 mg./lOO ml. 

 in Arenicola^'^ but negligible in Amphitrite and Nereis. Among Crustacea 

 the blood uric acid may be 2.3 mg./lOO ml. in Hoviarus, and 2.1 mg./lOO ml. 

 in Lihinia.^'^' In ureotelic mammals blood uric acid is 0.05 to 0.2 mg. 100 

 ml. ''" These values are much lower than in true uricotelic animals, as in 

 insects, in which blood uric acid is 5 to 20 mg./lOO ml.-^^- "" or in birds, in 

 which it is normally 5 mg./lOO ml. (chicken). '"" Hence the distinction 

 between ureotelic and uricotelic metabolism is quantitative, and presence of 

 uric acid in the blood or coelomic fluid does not necessarily indicate predom- 

 inant uric acid excretion. 



