SECT. 6] OF THE EMBRYO 901 



an adequate water-supply by coating their eggs with a watery 

 protein-containing mass. "Now it is hardly conceivable", said 

 Gray, "that the albuminous layer of the amnio te egg arose de 

 novo as an adaptation to terrestrial life, for this would involve a 

 sudden change in the structure of the oviduct. By the principle of 



Table 106. 



Survey of the movements of water in the hen's egg. 

 Water in grams 



& 2 Ji%^ ^%r 



II I 1 111 %tl Pt 



qI £ £ ill ►Sffs c^'s' 



o-o 8-5 29-9 o-o o-o 



o-oi 



\t 



0-4 8-45 27-2 2-4 



i-i 8-4 25-4 3-5 0-05 



2-5 8-2 23-0 4-6 0-I2 



4-6 7-8 20-4 5-6 0-27 



7'9 6-9 16-9 6-7 0-50 



I2-0 5-3 12-6 7-8 o-8o 



18 i8-i 2-3 9-2 8-8 1-20 



20 27-4 I-O 2-2 9-8 2-00 



7-5 27-7 Lost from yolk and white respectively 



35-2 Lost from both 



— — — 35-2 



9-8 



25-4 Lost other than by evaporation +2-00 by 

 synthesis =27-4 of which, even if the 

 whole of the water of the yolk went to 

 form the embryonic tissues (which is 

 unlikely) 20-0 gm. or 73 per cent, 

 would be water passing from egg- 

 white to embryo. 

 Compare this Table with Table 1 05 ; whereas the avian egg loses 

 some 25-5 per cent, of its initial store the chelonian egg gains some 

 40-0 per cent, from the exterior. For the question of bound water, 

 see p. 879. 



physiological continuity it is much more reasonable to suppose this 

 layer as equivalent to such homologous structures as are found in 

 the anamniota. Among fishes tertiary egg-membranes rich in water 

 are found in the Dipnoi, and they appear to be present in all am- 

 phibia. These membranes apparently protect the tgg against de- 

 struction by predatory animals though they may have subsidiary 

 functions associated with the incubation of the embryo. In most 

 amphibia, where the tertiary envelope is of a mucoid nature, the full 



