SECT. 9] PROTEIN METABOLISM 1143 



(Needham & Needham), would form a third class, but they are 

 notable exceptions in many ways, and to describe their behaviour 

 in terms of the language here used would be to say that they use 

 their own blood and yolk as the sea, and pile up the end products 

 of their protein metabolism within themselves. 



The amphibia never broke loose from the fishes; they always re- 

 tained a piscine larval stage and laid their eggs in water. But when 

 the first reptiles^ left the sea, they were faced with one or two very 

 difficult embryological problems. To begin with they had to find out 

 how to abandon metamorphosis, and to discover a way of arranging 

 a water supply for their embryos. As Gray has shown, aquatic embryos 

 depend upon their environment for a supply of water; in other 

 words, the fertilised egg contains enough solid, but not enough water, 

 to make the finished larva. The first terrestrial eggs, therefore, had 

 to contain enough water as well as enough solid, and, as arrange- 

 ments to prevent undue evaporation were essential, the closed-box 

 system inevitably developed. The mechanism by which a constant 

 pressure-head of water was provided in the terrestrial egg, namely, 

 the egg-white, can be seen functioning at the present time in the 

 yet unidentified acid, which, introduced by the embryo's metabolism 

 into the egg-white, as Vladimirov has shown, gradually brings the 

 latter to its isoelectric point, and liberates water by degrees from the 

 colloidal albumen. All the economy of the successful terrestrial egg had 

 to be directed towards conserving the water, and while a great bath 

 would have been required to keep the urea concentration down within 

 bearable limits, if all the nitrogen was excreted in that form, only 20 

 per cent, of the water in the egg need be set aside for handling uric 

 acid. Another way out of the diflSculty would have been to burn no 

 protein at all, and therefore to avoid all incombustible residues, and it 

 is possible that some of the extinct saurians explored the possibilities 

 in this direction, but I suspect that some factor which as yet we can- 

 not quite define dictates from within the cells that life without protein 

 combustion is not possible. It is therefore likely that such reptilian 

 experiments did not proceed very far. In this way the closed-box 

 system with its partial suppression of protein metabolism and its 

 uricotelic qualities came into being 



No doubt the first terrestrial vertebrates laid their eggs in water, 

 and probably, like the trout, they hatched early. But although this 



1 Or rather, their stegocephalic predecessors. 



N E II 73 



