Sgo 



GENERAL METABOLISM 



constant at about 85 per cent, during the last third of the first period 

 and the whole of the second period. The water-content of the yolk 

 is again uniformly low, about 60 per cent., and the fact that the 

 water-content of the whole system, embryo plus yolk, rises, is evidently 

 due to the fact that the embryo is growing all the time in size 

 relatively to the yolk which is diminishing in size. Where the water 

 of the embryo comes from is a matter which we may shelve for the 

 moment. Now there is a strong probability that the type of graph 

 shown in Fig. 232 for the trout may also turn out to be applicable 

 to the frog. Fig. 233 taken from Gray's paper, shows the decreasing 



Embryo <3> 



Embryo O 



Embryo+ Yolk* 



Yolk alone ® 



Embryo O 



Embryo+Yolk* 



Embryo + YolkJ 



Fig. 232. 



dry weight content of the larva (embryo plus yolk) in the frog and 

 the trout, so that, as far as that goes, they behave similarly. It was 

 said above that the frog larva is not capable of being separated into 

 its embryonic and yolk components, but this is not strictly true, for 

 a few careful dissections made by Glaser are available to throw 

 light on the matter. Glaser separated the embryos of Rana temporaria 

 into two parts, one predominantly consisting of yolk, the other of 

 "nervous tissue". Water estimations on these gave the figure of 

 54-2 per cent, for the yolk and 8o-i per cent, for the embryonic 

 body; these are plotted on Fig. 232, from which it can be seen that 

 the relations are just like those found by the workers on the trout. It 

 is, therefore, probably right to conclude that what happens both in 

 the frog and the trout is a constancy in water-content of the em- 



