GENERAL FACTS ABOUT SEA FISH 51 



capable of looking after itself, so in our fishes some leave the 

 egg little more than animated threads, while others (hatched in 

 most cases from heavy, adhesive eggs — e.g. the gobies) have 

 from their first days of freedom both red blood and an 

 open mouth. 



In sharks, the eggs and development of which differ in so 

 many respects from those of bony fishes, the young fish emerges 

 in a comparatively advanced condition. On the other hand, it 

 must be borne in mind that the young shark has been a long 

 time developing in the egg. Thus, whereas the eggs of some 

 bony fishes hatch out in a few days, the young of the black- 

 mouthed dog-fish {Pristiurus) is about nine months in the 

 egg ; that of the rowhound {ScylUum) about seven.* Conse- 

 quently, as might be expected, the baby shark has, by the 

 time it succeeds in rupturing the tough egg-capsule, acquired 

 all the characters of the adult, and it remains only for it to 

 increase in size and come to maturity. 



Far more wonderful in some respects is the early develop- 

 ment of young bony fishes, which may hatch from the egg in 

 four days, a transparent and helpless creature, measuring perhaps 

 -g- in., yet may acquire all the characters of the adult while 

 still less than \ in.f The yolk-sac, large and conspicuous, 

 and generally globular at the time of hatching, rapidly dwindles 

 day by day and disappears in less than a week, by which time 

 also the mouth has appeared and the fins are outlined. Spots 

 and dashes of colour-pigment may appear during the first few 

 days, and these will rapidly increase and spread. The special 

 metamorphoses of young flat-fishes, including general twisting 

 of the head and the passage of one eye either over the ridge of 

 the head or through its soft tissues, together with the disappear- 

 ance of colouring matter from the side bereft of its eye, will 

 be more appropriately alluded to in the chapter on that group 

 of fishes. 



* Cf. Bashford Dean, op. cit, p. 217. 

 t 3id., p. 225. 



