SECT. 6] 



OF THE EMBRYO 



875 



same period, (2) that some other gaseous product was eliminated, 

 or (3) that there was an increased evaporation of water. Neither of 

 the first two of these theories is very agreeable with the facts, for, 

 if ( I ) were true, the predominant substance combusted during the 

 last week would have to be protein, and we know almost certainly that 

 this is not the case, while all that is known about the metaboUsm of 

 the egg is against the second possibility (see the Section on respira- 



DbJj 



s 



5 6 



7 8 9 10 II 12 

 Incubation age 



Fig. 223. 



li 14 15 16 17 



19 2D 



tion). The third alternative must then be the correct one, and, 

 although it is not easy to see how the presence of the developing 

 embryo could increase the water elimination, it must be remembered 

 that the embryo is producing heat, and that evaporation would be 

 accelerated thereby. Probably the circulating blood in the allantoic 

 membranes, said Murray, is a more efficient evaporating mechanism 

 than the undifferentiated albumen. 



The dependence of water loss on external humidity demonstrated 

 by Murray shows that birds' eggs have no power of controlling the 

 amount of water they lose. This conclusion had been previously 



