SECT. 9] PROTEIN METABOLISM 1145 



They may first have discovered how to become permanently uraemic 

 without suffering from it, and then have utiUsed the associated ad- 

 vantage of protecting their embryos until a late stage of develop- 

 ment. Or they may have adopted a protective closed box, and then 

 become adapted in some way to withstand the consequent uraemia. 

 In any case, they offer an interesting comment on the terrestrial egg, 

 for they seem to have found out a way of avoiding the uricotelic 

 qualities of the closed-box system— a way, however, which appears 

 to have been only suitable for a very restricted class of animals.^ 



As for the Prototheria, their excretory nitrogen as Table 163 shows, 

 works out at a typically mammalian partition. As tho. Echidna is, strictly 

 speaking, oviparous, this would seem at first sight to be much in 

 opposition to the general views here suggested, but closer examination 

 shows that this is not so. The Echidna lays eggs, it is true, but, according 

 to Caldwell, who gives all the literature in his well-known paper, it 

 picks them up and immediately places them in its pouch. Once there, 

 they emerge at a very early stage from the thin shells, and suck up 

 the milk which exudes from scattered pores inside the pouch. There 

 seems no reason why the shells themselves should not be permeable 

 to excretory products (for they are not hard), and after hatching there 

 would not even be this difficulty — in both cases the epithelium of the 

 pouch would be able to absorb the foetal excreta. The pouch, in fact, 

 may be regarded as a uterus located in an unusual position, and, from 

 the standpoint of this discussion. Echidna would be viviparous, and 

 therefore "aquatic". It would be very interesting to investigate the 

 nitrogen partition in the urine of Ornithorhyncus which allows its eggs 

 to develop outside the body (Wood-Jones). 



A generalisation might then be provisionally enunciated as follows : 

 The main nitrogenous excretory product of an animal depends 

 on the conditions under which its embryos live, ammonia and urea 

 being associated with aquatic pre-natal life, and uric acid being 

 associated with terrestrial pre-natal life. Up to the present time there 

 has been no trace of order or system among the facts obtained by 

 comparative investigations on nitrogen excretion, and no answer to 

 the question of why a uricotelic metabolism should exist at all. The 

 answer here suggested is that terrestrial oviparous animals would 

 have been impossible without it. 



1 In this class the Dipnoi may perhaps be included. Smith has shown that the lung- 

 fish Protopteriis, aestivating in its burrow, accumulates relatively enormous amounts of 

 urea in its tissues. 



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