324 A HISTORY OF FISHES 



latter region. In all other Bony Fishes, as well as in the 

 Selachians, the amount of yolk present is usually so great 

 that it pre\'cnts segmentation altogether in the vegetative region 

 of the ovum, and cleavage is restricted to the small cap ot 

 protoplasm, the germinal disc, at the animal pole. There is, 

 however, no hard and fast line between the two types of seg- 

 mentation, certain species of fish providing transitional stages. 

 After the blastula has been formed, by the inpushing of one 

 side, this is converted into a double-walled cup, the gastrula, 

 the central cavity of which becomes the alimentary canal. 

 From this stage onwards growth proceeds rapidly, and each 

 of the cells of the two layers goes on dividing. Concurrently, 

 groups of cells become specialised for the performance of par- 

 ticular duties, and give rise to the various tissues and organs — 

 alimentary canal, notochord, brain, muscles, gill-slits, and so 

 on — the mouth and vent are formed, and the embryo comes 

 into being. 



The early stages of development just described may con- 

 veniently be termed embryonic, in contrast to the post-embryonic 

 or larval development which takes place after the egg is hatched. 

 Such a division of development into two stages is, of course, 

 merely arbitrary, and the whole process from the fertilised egg 

 to the mature fish goes on without a break. In a large number 

 of fishes the development may be almost entirely embryonic, 

 the young fish, when hatched, being a repUca of its parents, 

 except in size. In others, on the other hand, the young fish 

 is obliged to leave the shelter of the egg before it has reached this 

 stage, and has to undergo a further series of changes, con- 

 stituting the larval development, before its bodily structure is 

 that of a mature fish. There is an important relation between 

 the size of the egg and the condition of the young fish on 

 hatching. As a general rule, in those fishes producing large 

 eggs the young are hatched in a fully developed condition, and 

 have little, if any, larval development. In those with small 

 eggs, on the other hand, the post-embryonic or larval stage is 

 more or less prolonged. This relation is clearly connected with 

 the amount of yolk present in the egg. In the case of the 

 oviparous Selachians, with their large and heavily yolked eggs, 

 the abundance of food enables the embryo to remain within 

 the egg for a long period, and when hatched it has reached an 

 advanced stage of development. Thus, a Black-mouthed Dog- 

 fish (Pristiurus) hatches out about nine months after fertilisation, 

 a Spotted Dog-fish [Scyliorhinus) about seven months. In the 



