iv EGGS AND LARVAE AND THEIR DEVELOPMENT 99 



the life of the individual fish to commence the moment the egg 

 from which it arises is fertilised. This life consists of four distinct 

 stages or periods, of which the stage of development in the egg is 

 the first. The second stage commences after the fish is hatched. 

 At this time it is not like its parents, but very different, and is 

 usually called a larva. When a butterfly is first hatched it is a 

 caterpillar, a very different creature, and it has to go through a 

 transformation before it becomes a butterfly. The fish likewise 

 goes through a transformation, and the stage from hatching to 

 the completion of this transformation is the second period of its 

 life. The fish, however, after its transformation is still very small 

 and is not mature, its generative organs are not fully developed, 

 and therefore it does not begin to breed. The period from its 

 transformation to its maturity is the third period of its' life, and 

 the fourth is that of the mature adult condition which is 

 terminated sooner or later by death. 



The development of the egg of the bony fish differs from 

 that of the dog-fish or skate in many important matters, but there 

 is a general similarity between the two cases. In the bony fish 

 the duration of the process is much shorter, and the fish when 

 hatched is in a much less perfect condition. In fact in the case 

 of the dog-fish the first and second periods of life just defined 

 are not separate at all, but both included in the period of 

 development inside the egg-shell. This will be more fully 

 explained when the larva and its transformation are described. 



The first step in the development of the bony fish is the 

 division of the mass of germinal matter into portions which remain 

 connected. It is by the continual division and growth of these 

 portions that the development of the fish is brought about. The 

 germinal matter is thus converted from a single mass of sub- 

 stance into a compound " tissue " from which the flesh and 

 organs of the embryo arc formed. At first the tissue becomes 

 thin and forms what may be called the germ-skin, which spreads 

 out over the surface of the yolk until it completely encloses it. 

 But from about the centre of this germinal membrane to its 

 edge there is a thickened rod, which marks the back of the 

 young fish (Figs. 51, 52). The inner end of this rod, that which 

 is in the centre of the germinal membrane, is thicker than the 

 rest, and this part is the commencement of the fish's head. At 

 the sides of this part appear two circles marking out the future 



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