THK HE1WINO FAMILY. 418 



digested and absorbed, and the indigestible residue got rid of. 

 so that the organs of the alimentary canal, jaws, oesophagus, 

 stomach, liver and intestine come into full play at this stage. 

 In fact, in the embryonic and larval stages the fish is self- 

 contained as regards its food-supply ; on the assumption of the 

 post-larval condition this state of affairs is altered and it 

 becomes directly dependent in this respect upon its external 

 environment. 



This is true in the case of the transition from embryo to 

 larva only as far as the sensory and locomotory organs are 

 concerned. The accomplishment of hatching allows a more 

 direct interaction between the muscular and sensory systems and 

 their environment, but the larva is, as stated above, still com- 

 pletely independent of the latter in respect of alimentation. 

 These considerations will serve to emphasise the point that the 

 period of ' incubation ' as measured by the lapse of time from 

 fertilization to hatching is neither theoretically nor practically 

 of specifically fixed duration. In the chapter on Rate of Growth 

 this subject is further dealt with. 



The newly hatched herring (Plate XIX, fig. 2) has been 

 described and figured by several observers. As is seen in the 

 figure, it is an attenuated form and the whole body is colourless 

 and transparent. In several features this larva is more ad- 

 vanced than that of the sprat, thus e.g. the mouth already 

 gapes widely, and the eyes are deeply pigmented. From an 

 examination of freshly caught larval herrings in this Laboratory 

 (St Andrews) undertaken to decide this point, there can be no 

 doubt that the eyes of the larval herring are deeply pigmented 

 and have, in life, a bluish-green metallic lustre. Holt has 

 previously stated them to be of a brilliant silvery blue, whilst 

 Kupffer affirms that they are black. Black pigment is present 

 and can be seen at the outer edge of the iris and in the pupil ; 

 and the eyes of preserved specimens have a uniformly black 

 colour. There can be little doubt that Cunningham's statement 

 that the eyes are colourless and devoid of pigment is incorrect. 

 In his recent book upon Marketable Marine Fishes, he agrees 

 with Kupffer's statement. 



The alimentary canal does not terminate immediately behind 



