412 THE HERRING FAMILY. 



it collects as a mass at one pole and forms the blastodisc. 

 Segmentation of the blastodisc then proceeds in the usual way. 



Development is rapid and by one and a-half days the 

 embryo reaches three-quarters round the yolk. On the fourth 

 day it shows sigus of movement. On the fifth, the hind gut opens 

 to the exterior, and there is little change from the 6th to the 

 7th days, on the latter of which the embryo hatches (Kupffer). 



The length of the newly hatched larval herring is given by 

 Kupffer as 52 53mm., whilst Hoffmann found that his larvse 

 were 6'2 6.4 mm., and Boeck again records newly hatched 

 larvse of 10 mm. It should be borne in mind that Boeck found 

 the North Sea herring's egg to be considerably larger than 

 those of the Baltic and other parts. Newly hatched larval 

 herrings occur every year in St Andrews Bay in early March, 

 and they are usually about 7 mm. in length. The difference in 

 size between the larva? of Kuptfer and Hoffmann may be ac- 

 counted for by the variable duration of incubation: seven days in 

 Kupffer s case, twelve days in that of Hoffmann. This has been 

 clearly shown by Meyer, as indicated above, for he was able to 

 vary the period of incubation by a change of temperature, 

 within the limits of 8 to 40 days, and by this means obtained 

 larvae varying between the limits of 5'4mm. and 9 mm. In the 

 nature of the case, there is no inherent reason why the period 

 of hatching should be fixed and definite, all the processes of 

 development being uninterrupted by, and independent of, its 

 occurrence. (See chapter VI, on Rate of Growth.) 



The point of transition from the larval to the post-larval 

 stage is a more clearly defined epoch in the career of the 

 young fish, involving as it does many important physiological 

 and morphological changes. It is at this time that the young 

 fish really commences its independent existence, for the supply 

 of ready-prepared nutriment or yolk then comes to an end and 

 thereafter it has to depend upon its own exertions for obtaining 

 the requisite food. By a reference to the general account of the 

 development of these fishes it will be clear that the yolk-nutri- 

 ment is made available for the recuperation of the tissues 

 through the medium of the blood-supply, whereas the organisms 

 which form the post-larval food require to be caught, ingested, 



