CHEMICAL STRUCTURE AND METABOLISM 93 



is added to the feces, and the water is reabsorbed into the body in the 

 posterior portion of the digestive tract. In many Hzards, snakes, and birds 

 the urine is a semisoHd mass of uric acid crystals, excreted with almost no 

 loss of water. 



It is interesting that some turtles have been found to excrete nitrogenous 

 wastes both in the form of urea and in the form of uric acid. Since tur- 

 tles are a very ancient group of reptiles (see Fig. 9.3, p. 177) this fact 

 may indicate that early reptiles could accomplish both types of excretion. 

 At any rate, the urea-forming system was doubtless inherited from am- 

 phibian ancestors. The latter also doubtless passed on the same method of 

 excretion to the reptiles that were the ancestors of mammals (Fig. 9.3). 

 Apparently the uric acid system was the "new development" which even- 

 tually came to characterize more highly specialized reptiles (e.g., lizards 

 and snakes) and birds. 



Needham has stressed the importance of embryonic needs in the devel- 

 opment of the two systems of nitrogen excretion. From our discussion of 

 the human embryo and its contact with the mother's blood stream we ap- 

 preciate that urea formed in the embryo is readily transferred to the moth- 

 er's blood and excreted by her kidneys. Things are quite otherwise for 

 reptile and bird embryos. Locked away in their eggshells (Fig. 8.24, 

 p. 169) these embryos must "live with" their waste products until hatch- 

 ing time. Tissues of most animals cannot thrive in a high concentration 

 of urea. Nitrogenous wastes in bird embryos are converted into insoluble 

 uric acid and then stored in the embryonic sac known as the allantois. 

 In this manner the tissues are freed of soluble nitrogenous wastes that 

 might interfere with normal metabolism — clearly an adaptation to the 

 needs of life within an eggshell. How the turtles with their urea excretion 

 fit into this picture is not clear. Some turtle eggs are laid in moist sand; 

 possibly urea in solution can be passed out through the eggshell under such 

 conditions. The matter merits further investigation. 



At this point mention is appropriate of a sequence of events in the chick 

 embryo that has frequently been cited as an example of biochemical re- 

 capitulation. Needham (1931) pointed out that in this embryo the total 

 amount of ammonia relative to dry weight of the embryo reaches a peak 

 on about the fourth day, that the relative concentration of urea is highest 

 on about the ninth day, following which the concentration of uric acid be- 

 comes maximal. These facts have been interpreted to indicate that the 

 chick embryo exhibits biochemical recapitulation: first excreting am- 

 monia like a fish, later excreting urea like an amphibian, and then attain- 

 ing the uric acid excretion which will characterize it as an adult. Reinves- 



