132 Embryology and Growth of Fishes 



an inner layer of cells and an outer layer, the inner layer 

 being known as the endoblast and the outer as the ectoblast. 

 The cavity of the cup thus formed is the archenteron and gives 

 rise primarily to the alimentary canal. This third well-marked 

 stage is called the gastrula stage, and it is thought to occur 

 either typically or in some modified form in the development 

 of all metazoa, or many-celled animals. In the lampreys, the 

 Ganoids, and the Dipnoans the eggs contain a much greater 

 quantity of yolk than those of the lancelet, but the segmenta- 

 tion resembles that of the lancelet in that it is complete; that 

 is, the whole mass of the egg divides into cells. There is a great 

 difference, however, in the size of the cells, those at the upper 

 pole being much smaller than those at the lower. In Petromyzon 

 and the Dipnoans blastula and gastrula stages result, which, 

 though differing in some particulars from the corresponding stages 

 of the lancelet, may yet readily be compared with them. In 

 the hagfishes, sharks, rays, chimaeras, and most bony fishes there 

 is a large quantity of yolk, and the protoplasm, instead of being 

 distributed evenly throughout the egg, is for the most part ac- 

 cumulated upon one side, the nucleus being within this mass of 

 protoplasm. When the food substance or yolk is consumed and 

 the little fish is able to shift for itself, it leaves the egg-envelopes 

 and is said to be hatched. The figures on page 135 show 

 some of the stages by which cells are multiplied and ultimately 

 grouped together to form the little fish. 



Post-embryonic Development. In all the fishes the develop- 

 ment of the embryo goes on within the egg long after the gastrula 

 stage is passed, and until the embryo becomes a complex body, 

 composed of many differing tissues and organs. Almost all the 

 development may take place within the egg, so that when the 

 young animal hatches there is necessary little more than a rapid 

 growth and increase of size to make it a fully developed mature 

 animal. This is the case with most fishes: a little fish just 

 hatched has most of the tissues and organs of a full-grown fish, 

 and is simply a small fish. But in the case of some fishes the 

 young hatches from the egg before it has reached such an ad- 

 vanced state of development, and the young looks very different 

 from its parent. It must yet undergo considerable change 

 before it reaches the structural condition of a fully developed 



