898 GENERAL METABOLISM [pt. iii 



of the lana soon reaches the volume of the original egg-shell ; further 

 development without hatching would crush the embryo against the 

 egg-membranes. In aquatic animals a postponement in the date of 

 hatching is only possible when either the original egg-membrane is 

 highly elastic or when it is separated from the yolk by a wide peri- 

 vitelline space. Now an aquatic egg containing sufficient perivitelline 

 space to allow the act of hatching to be postponed until the young 

 organism is fully formed (and comparatively unhampered by re- 

 maining yolk) would form just the system most easily adaptable to 

 terrestrial habits. Such an egg laid on land would undergo its normal 

 development as long as it was protected from undue evaporation 

 during the incubation period. Thus the experimental data we have 

 point strongly to the suggestion that the Reptilia and their derivatives 

 arose from a type of organism whose eggs were laid in water 

 but which did not hatch until the yolk-sac period of development 

 had reached an advanced stage. The wide perivitelline space which 

 was then a necessary feature for the accommodation of the enlarging 

 embryo became on land the means whereby water was supplied for 

 development. The significance of the white of the hen's egg can be 

 realised by the fact that after 19 days of incubation the embryo has 

 absorbed i o cc. of water from it. The eggs of all birds and of many 

 reptiles are of the same type, but in some reptiles the necessary 

 water is partially supplied from external sources, and the eggs swell 

 appreciably after being laid" (e.g. Sphenodon — Dendy; Dermochelys — • 

 Deraniyagala; and many others). 



A remarkable illustration of this is suppliedby the workof Karashima 

 on the Japanese marine turtle, Thalassochelys corticata, which lays eggs 

 the size of pingpong balls in the damp sand above high-water mark. 

 He did not himself calculate the movements of water in these eggs but 

 this can easily be done from his figures, with the following result : 



