THE INFANCY OF FISHES 209 



what is true of the shark tribe is true also, as we shall 

 presently show, of other groups of fishes. 



As we have already pointed out, when the food store 

 is large the embryonic period is prolonged, and in the 

 case of the fishes now under consideration the larval stage 

 is further commonly passed before emergence from the 

 sheltering egg-shell. The hatching, indeed, of the young 

 of the shark and dog-fish tribe is commonly long deferred. 

 In some of the dog-fish, as for example in Pristiurus, it 

 does not take place till the end of the ninth month of 

 incubation : in the common dog-fish at the end of the 

 seventh month. 



It will be remembered that the larvae of the common 

 frog are hatched in a very premature condition, being 

 mouthless, limbless, and blind ; but that in a few 

 days external gills make their appearance. In the young 

 sharks and dog-fish these external gills are developed 

 and absorbed before emergence from the shell, as may 

 be seen in the accompanying illustration, showing the 

 larva of the picked dog-fish. But it is remarkable to 

 find that the food store is so enormous that at the time 

 of hatching a large amount has still to be absorbed. The 

 little fish, as is shown in our illustration, has attained the 

 fully developed adult form, but suspended from its ab- 

 domen is a large sac of still unabsorbed , yolk. This 

 performs the dual and remarkable function of larder 

 and anchor, for by its weight the young is held down to 

 the bottom of the sea, a prisoner until the process of 

 digestion is completed ! 



And a precisely similar state of affairs obtains in the 

 case of the curious eel-like gymnarchus, of the Nile, 

 wherein the yolk sac is drawn out into an enormous 

 sausage-shaped body. Here, it will be noticed, hatching 



