SECT. 6] 



OF THE EMBRYO 



893 



ment by combustion, and a good deal of the eventual water is taken 

 in from outside. Now between a quarter and a third of the initial 

 dry material is so used, so that the eventual dry weight in the finished 

 fish is about 25 mgm., which at 84 per cent, water needs 155 — 25, i.e. 

 130 mgm. water. But, as there were only 59 mgm. water present 

 in the yolk at the beginning, 130 — 59, i.e. 71 mgm., must have been 

 absorbed from the exterior. These relationships are shown in the 

 chart, constructed by Gray and given in Fig. 236. It will be seen at a 



45 



10 20 30 40 50 60 70 80 

 Days after Fertilisation 

 Fig. 235. 



90 100 no 



glance that the newly fertilised tgg contains enough solid for construct- 

 ing the finished embryo and for providing the material for combustion 

 to serve the basal metabolic requirements, but it does not contain even 

 half enough water. The latter has, therefore, to be taken from the 

 aqueous environment, according to the following generalised equation : 

 + 



Wet 



yolk 



(i-ogm. 



External 



water 

 (0-7 gm.) 



Wet 



fish 



(1-56 gm.) 



Dry yolk used for 



other purposes 



(o-i4gm.) 



By respiration experiments, as already remarked. Gray was able to 

 account for all the yolk disappearing in the last of these fractions. 



In a later paper Gray found that a peak exists in the wet weight 

 of the larva (yolk plus embryo); this is illustrated in Fig. 237. Up 

 to the 85th day the wet weight of the larva increases, but after that 

 time it falls, although the wet weight of the embryo steadily in- 

 creases throughout the period. Thus after the 85th day the yolk is 



