TECHNICAL NOTES AND BIBLIOGRAPHY 2/1 



respiratory purposes when the animal is submerged, and 

 it would not be surprising if urea is excreted by simple 

 dijBFusion into the fluid contained in these sacs, a cir- 

 cumstance that would favor the urea habitus and that 

 may also have distorted the recorded data on urinary 

 nitrogen. Anal sacs are apparently not present in the 

 marine turtles, but urea may be lost through the oral 

 membranes. 



The alhgator (A. mississippiensis) is likewise a fresh- 

 water animal, and hkewise an exception to the uricoteHc 

 rule; but urea is present, if at all, in the blood and urine 

 only in traces. The chief nitrogenous constituent of the 

 urine is ammonia, which is excreted with large quantities 

 of bicarbonate. But the alhgator remains a reptile, and 

 of what nitrogen it does not excrete as ammonia, the 

 greater part is excreted as uric acid {54, p. 194; 132}. 



Needham {67} continues in his earlier belief that the 

 form in which nitrogen is excreted depends primarily 

 on the conditions under which the embryo has to Hve, 

 applying this principle to the elasmobranchs, which, 

 having first developed severe uremia (the reason for 

 the uremia not being stated), were then able to enclose 

 the embryo completely, in order to promote (in an im- 

 specified way) its advanced development. He argues in 

 a parallel manner that, in the birds, uric acid metabohsm 

 is an adaptation to the cleidoic (closed) egg, since the 

 accumulation of urea (but not uric acid) would pre- 

 sumably have pathological effects. This interpretation 

 with respect to both the elasmobranchs and the birds 

 (and reptiles) appears to the vmter as an inverted read- 

 ing of the facts, and our earher reply {69} seems to 

 require no amendment. 



To Needham's recent statement that 'No other gen- 

 eralization will explain the lack of uricoteUsm (uric 

 acid excretion) among mammals, a plainly terrestrial 

 class* {67, p. 237 footnote 1}, it can be replied that no 

 generalization is needed other than biochemical conti- 



