SECT. 6] 



OF THE EMBRYO 



907 



yoLk-sac period -*« hunger— 



provisionally it may be accepted. Fig. 239 gives a graph plotted from 

 their data. The percentage growth-rate falls markedly at the end of 

 the first period, only to rise again to a maximum during the free- 

 swimming period. Everything, in fact, points to a resumption of 

 the growth-process as soon as an unlimited supply of water is avail- 

 able, which will only fall off again when the nutriment in the yolk-sac 

 is beginning to be exhausted. Kronfeld & Scheminzki pointed out 

 that the decline in the growth-rate before hatching occurred before 

 all the water in the yolk was finished, as would be expected in view 

 of the osmotic pressure of the 

 latter. They drew attention to ^ 

 Schaper's well-known experi- 5 

 ment on frog embryos and J 

 larvae, in which he placed them J 

 in salt solutions, and, by thus I 

 holding back the water which | 

 they needed to absorb to form | 

 their tissues, succeeded in in- ^ 

 hibiting their growth. Kronfeld % 

 & Scheminzki confirmed this ^ 

 for the trout in some preliminary 

 experiments. The question of 

 whether growth of the trout fry 

 can continue after the yolk has 

 been used up was left open by Kronfeld & Scheminzki, but Weiss had 

 previously maintained that this could occur, and Podhradski & Kosto- 

 marov confirmed Weiss' results on carp alevins at the end of their 

 yolk-sac period. It must be supposed that this growth is either due 

 to the persistence of small amounts of yolk which gradually get used 

 up after the yolk-sac has apparently disappeared, or to the utilisation 

 of muscles or other tissue as nourishment instead of yolk. The latter 

 process is suggested by the work of Krzinecki & Petrov^. 



The process of water-absorption by the amphibian larva during 

 its development was studied also by Bialascewicz, who used the un- 

 satisfactory but relatively easy method of measuring volume changes. 

 In all his curves, a temporary reduction of volume is seen about 



10 20 30 40 50 60 70 80 90100110120 



(0% growth rate wet weight) Embryo 

 KiS<* ° " " "'O' " i 



)0 °/» de-growth „ wet .• 1 Yolk 

 l.<j> % r, „ „ dry ., / 



Fig. 239. 



^ In certain cases, the intake of water by the egg before hatching will mask the loss 

 of dry solids by combustion, if the eggs are weighed in air. Weighing in water, as 

 suggested by Ritter & Bailey, may therefore be a useful method. 



