SECT. 6] OF THE EMBRYO 899 



This table demonstrates {a) that the chelonian yolk absorbs water 

 from the white, just as does that of the bird — a process which will 

 tend to dilute the yolk appreciably if it goes on faster than the forma- 

 tion of embryo and amniotic liquid; and {b) that 1136 gm. of water 

 are absorbed from the exterior by 100 eggs, i.e. 42-0 per cent, of 

 the original amount provided by the maternal organism, which thus 

 expects its embryos to obtain for themselves about a third of the 

 water they require. A swelling of the egg must certainly have taken 

 place, though Karashima did not report it : and this lack of water 

 may in part account for the relatively small size of many chelonian 

 eggs. 



Still more remarkable is the fact, reported by Cunningham, that 

 another turtle (from North Carolina), Chrysemys cinerea, has a special 

 mechanism for wetting the earth in which the eggs are to incubate. 

 "These turtles", says Cunningham, "select high ground in which 

 to build their nests, sometimes a considerable distance from water. 

 Usually the ground chosen is hard and dry, but a sandy beach may 

 be used. The dirt is first moistened by water from a supernumerary 

 bladder; as is shown by the fact that the dirt in the hole and sur- 

 rounding it is wet while that further away is hard and dry." Cunning- 

 ham analysed the water in this bladder (the function of which had 

 previously been unknown) and found it to be a dilute urine, con- 

 taining only 0-0005 P^r cent, of nitrogen. During development the 

 eggs swell somewhat. This is a notable link in the evolutionary chain, 

 for here the turtle goes out of its way to provide a store of water for 

 its terrestrial eggs, yet outside not inside them. This must be the furthest 

 point to which a non-cleidoic egg could go in a terrestrial environ- 

 ment (see p. 1 103). Moulton states that terrapin eggs have been com- 

 mercially incubated by putting them in sand and sprinkling the 

 surface every week until hatching, and Hochstetter reports success 

 by a similar method on Emys europaea. Deraniyagala and Hildebrand 

 & Hatsel find that loggerhead turtle's eggs {Caretta) take up a good 

 deal of water during incubation. "Apart from other advantages". 

 Gray continued " the value of the invention of viviparity is obvious; it 

 entirely removes the necessity for the parent organism to provide in 

 the newly fertilised egg enough water to last the embryo throughout 

 the whole developmental period. Now it is known that the vitelline 

 membrane in the hen's egg is permeable to water but not to salts 

 and other osmotically active substances. Were the yolk surrounded 



