SECT. 6] OF THE EMBRYO 903 



This 300-fold increase in volume must largely be due to absorption 

 of water, though a certain increase in dry weight may well occur. 

 The only significant difference between an egg of a monotreme and 

 an egg of a reptile is that, in the former, the aqueous secretions of 

 the walls of the oviduct are passed straight into the yolky ovum 

 itself instead of being deposited on its surface as a separate phase. 

 In eutherian mammals this process has gone one step further since 

 the water contained in the mother's blood is passed, not into the 

 ovum, but direct into the embryo. 



"If these arguments are sound", Gray continued, "there seems 

 good evidence to show that terrestrial vertebrates have descended 

 from a fish-like ancestor which possessed a glandular oviduct. The 

 secretions of these oviducts were at first utilised as a protective covering 

 to the eggs, but eventually they made it possible for the eggs to develop 

 on land by providing an adequate supply of water to the embryo." 



Gray's contention that there exists an evolutionary continuity 

 between the amphibian egg-jelly and the avian egg-white, that they 

 are, in fact, homologous structures, acquires still further interest from 

 an isolated observation reported by Banta & Gortner, In the course 

 of their work with Amblystoma, they found one day a mass of eggs 

 contained in an opaque milky white jelly instead of the usual trans- 

 parent and translucent material. Investigation showed that the dry 

 weight of the normal jelly was 337 mgm. per cent, and that of the 

 milky white one 361 mgm. per cent. When desiccated the jellies were 

 indistinguishable in appearance but swelled up again to their original 

 condition. The phenomenon was not, as far as could be ascertained, 

 due to bacteria, and as the milky jelly has 9-18 per cent, nitrogen 

 (dry weight) as against the 8-32 per cent, of the normal kind, they 

 thought that perhaps the former might consist mainly of albumen 

 instead of mucin. This was confirmed by qualitative tests. Here then 

 was an aberrant example of an amphibian egg-jelly resembling the 

 avian and reptilian egg-white, and indeed only requiring a shell to be 

 transformed into it^. It affords an interesting commentary on Gray's 

 remarks. The suggestion of Steudel & Osato mentioned on p. 331 

 may also be recalled — they pointed out that mucoprotein is not 

 absent from the egg-white even of birds, and expressly referred to 

 an evolutionaiy continuity with amphibia. The land-frogs offer some 



1 Shells of a rudimentary kind do occur in amphibia (e.g. the land-frog Rana opisthodon 

 and the African toad Xenopus laevis (Bles) ) . 



N E II 58 



