900 GENERAL METABOLISM [pt. iii 



by pure water, the latter would rapidly pass into the yolk and thereby 

 produce a large and mechanically weak ovum. If the aqueous 

 surroundings, on the other hand, consisted of crystalloid substances 

 the ease with which the embryo could obtain water would become 

 increasingly less with increasing age owing to the rising osmotic 

 concentration of the external salts. Since, however, the initial 

 osmotic equilibrium between embryo and yolk on the one hand, 

 and the aqueous surroundings on the other, is effected by means of 

 a colloid, then although water is withdrawn from the latter its 

 osmotic pressure does not rise as much as would that of a solution 

 of a crystalloid. The existence of an albuminous solution round the 

 yolk of terrestrially developing eggs appears to be an admirably 

 adapted mechanism for providing the growing embryo with water." 

 Thus Gray's theory amounts to this, that we may see in the egg- 

 white of the hen's egg a mechanism for supplying the embryo with a 

 relatively constant pressure-head of water. If the chick embryo were 

 dependent on the yolk alone, it would never be able to construct its 

 tissues, for the yolk has only about 45 per cent, of water and the 

 chick about 80 per cent, at the time of hatching. It is hardly 

 necessary to indicate the way in which the findings of Vladimirov, 

 referred to above (p. 880), fit in with the general viewpoint of Gray, 

 and it looks very much as if the acid which seems to be produced by 

 the embryo and which appears in the white, is the regulatory or 

 control device governing the transfer of water from egg-white to 

 embryo. 



Table 106 gives a succinct survey of the movements of water in 

 the hen's egg; it was calculated by Gray from the experimental data 

 of Murray. It clearly appears that at least two-thirds of the water 

 in the finished chick embryo is derived from the albumen. 



In a later paper Gray continued his exposition of the relations 

 between the water metaboHsm of the embryo and the evolution of 

 terrestrial vertebrates. Agreeing with Watson that the main problem 

 of evolutionary modifications centres round the possibility of deri\ing 

 one morphological type from another without requiring any func- 

 tional discontinuity of the organs involved, he considered the origin 

 of the egg-white in birds. No reptilian or avian egg exists without 

 an albuminous phase, and we may assume that its function is not 

 radically different from that of the egg-white of the chick. The 

 amniota, then, solved the problem of providing their embryos with 



