9o6 



GENERAL METABOLISM 



[PT. Ill 



67. Water-metabolism in Aquatic Eggs 



Fig. 238 taken from Kronfeld & Scheminzki's paper illustrates once 

 again the relations we have been discussing in the case of the trout 

 egg. It shows that the maximum intensity of water absorption occurs 

 about half-way through the yolk- 

 sac free-swimming period. The 

 total water in the larval system 

 rises steadily from fertilisation 

 onwards and also that in the 

 embryo, but there is a distinct 

 loss of water from the yolk. This 

 does not mean that the yolk 

 becomes drier, but is simply a 

 measure of the disappearance 

 of the yolk. Before hatching the 

 loss of water from the yolk is just 

 compensated for by the gain in 

 water of the embryo, showing 

 that at first the system is a closed 

 one. There is here a certain con- 

 trast with the frog's egg, for the 

 water content of the embryo 

 plus yolk there begins to rise 

 well before hatching, though 

 Gray asserts that water-absorp- Water in the yolk. 



tion begins before hatching in Fig. 238. The vertical lines indicate relative 

 , T i_ 1 intensities of water absorption, and the 



the trout too. In the lump- asterisks the maxima of dry and wet weight 



sucker {Cyclopterus) Hayes found respectively. 



no intake of water until after hatching, but in the Atlantic salmon 

 [Salmo salar) there was a definite rise of about 10 per cent. 

 Kronfeld & Scheminzki fully appreciated the fact that the trout egg 

 contains enough solid but not enough water to make the embryo, and 

 they attempted to show that at the end of the first period a slowing of 

 the growth-rate was perceptible, owing to the inability of the embryo 

 to get sufficient water through the egg-membranes to dilute its solid 

 material. The figures on which they based their growth-rate estima- 

 tions, however, were rather too few to substantiate this; nevertheless, 

 some statements in Gray's paper appear to coincide with it, and 



Water in the larva. 

 Water in the embryo. 



