CHAPTER II. 



LIFE-HISTORY AND DEVELOPMENT OF A FISH FROM 

 A PELAGIC EGG. 



IN the roe or ovary the egg of the fish is developed from a 

 minute cell quite invisible to the unaided eye to a size that 

 can readily be observed. During this process many important 

 changes go on both with regard to the egg and its surroundings, 

 and amongst others is the change from an opaque mass to a 

 transparent glassy egg at maturity. This is due to the dis- 

 appearance of the granules of the yolk which fills the egg, and 

 of the contained bodies (nucleus and nuclcolus), together with the 

 formation of the female pronucleus. Moreover, at this time the 

 egg increases considerably in size, especially in the pelagic forms, 

 and hence the advantage of the provision whereby only a certain 

 number assume this condition at a given period. If the whole 

 of the eggs of such a fish, for instance as the cod, ripened 

 simultaneously the body of the fish would be ruptured. The 

 condition therefore is different from that in such fishes as the 

 herring, wolf-fish, lump-sucker, sand-eel and salmon, in which the 

 majority of the eggs ripen simultaneously and are shed about 

 the same time. 



The translucent eggs float about in the water, rising near the 

 surface wherever the adults have shed them, and many are 

 remarkably hardy. Thus the late Prof. Huxley having suggested 

 in 1884 that perhaps the floating or sinking of the eggs was a 

 question of temperature, a series of eggs of the flounder which 

 had been fertilized on the 2nd May were placed in a test-tube 



