IV EGGS AND LARV^ AND THEIR DEVELOPMENT lOI 



eyes, and behind these a cavity on each side becomes the ear. 

 After the yolk has been completely enclosed by the germinal 

 membrane the little "embryo" still goes on growing at its 

 hinder end, and the result of this is the formation of the tail 

 (Fig. 53)- 



When the buoyant egg is left to itself in still water it alwa}'s 

 floats with the germ or germ-skin downwards, because this is 

 heavier than the yolk. It should be noted that eggs do not 

 float because they have an oil-globule, since in a very large 

 number of floating eggs there is no separate oil-globule : they 

 float because they are lighter than sea-water, and they do not 

 float in fresh water, nor when they are unripe, nor when they 

 are dead. 



The intestine, including under that term the greater part of 

 the food-pipe, namely gullet, stomach, and intestine, is formed as 

 a simple uncoiled tube. When development has reached this stage 

 it is easy to see that the yolk is now a large mass within the belly 

 of the little fish, which is greatly swollen by it. The thickened 

 rod is formed by, or becomes in the process of growth, the flesh 

 or muscles of the back of the little fish, and in the centre of these 

 is a slender rod around which the bones of the spine are after- 

 wards formed. There is no mouth. The fins arise as simple 

 flaps of skin without bones ; one of these in the middle line runs 

 along the back round the end of the tail, and forwards again 

 beneath the latter to the yolk. The breast fins appear as a 

 delicate flap on each side behind the head : the hinder fins do 

 not appear till later. 



In the condition now described, in the case of buoyant 

 separate eggs, the little fish is hatched, that is to say the egg- 

 membrane bursts because it is no longer strong enough to resist 

 the growth and movement of the tail, and the little larva is set 

 free (Fig. 54). It is not difficult now to understand why the fish 

 when first hatched is called a larva, to mark its difference from 

 the fully developed fish. It has no mouth to feed with, no red 

 blood, and no bones. At first it floats upside down, because 

 there is still so much yolk, and the yolk is lighter than the body. 

 It goes on developing, nourishment as before being provided by 

 the yolk. On the newly hatched fish there are little stars of 

 colour, the tint and arrangement of which are different in 

 different species. These are the pigment-cells which are formed 



