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CHAPTER IV. 



BREEDING. 



THE egg of any particular species of fish is derived from a female individual of that 

 species, and after a process of development becomes another individual of that species, 

 presenting the characteristic specific characters. As the eggs of nearly all marine fishes 

 are left to themselves after being shed from the body of the female, the parents taking 

 no care of them, there are only two possible methods of ascertaining the species to 

 which a particular kind of egg belongs, or of tracing the development of any particular 

 species of fish. One method is to observe the deposition of the eggs by the female, or 

 to detain them by artificial fertilisation, then to examine these eggs and study their 

 development. The other is to examine all kinds of eggs that can be obtained, and to 

 trace their development up to the stage when the young fish presents specific charac- 

 ters by which it can be identified. The latter method presents more difficulties and 

 uncertainties than the former. 



The greater number of marine fishes shed eggs or spawn at one particular period of 

 the year, a period extending over one or a few months. During the rest of the year 

 the development of ova in the ovaries proceeds gradually until the next annual breeding 

 season, when the annual crop of mature ova is again shed. As the ova develop in the 

 ovary they increase greatly in size and number, and thus the ovary itself becomes 

 much enlarged, and finally attains a very considerable size in proportion to the rest of 

 the body of the female. This enlargement of the ovary produces a corresponding 

 enlargement of the visceral region of the female. The small testes do not exhibit any 

 corresponding increase in size in the sole. In most fishes the testes in the male enlarge 

 at the breeding season in some degree, and in some they become very nearly as large 

 as the ovaries in the female. In the herring, for instance, it is not possible to distinguish 

 the males from the females among ripe specimens taken from the net when captured, 

 except by squeezing them and observing whether eggs or milt escape from the genital 

 aperture. But in the sole the ripe females can be easily distinguished by the enlarge- 

 ment of the ovarian region. It is not so easy to distinguish the smaller males from 

 small immature females, but among larger specimens the males can usually be identi- 

 fied by holding the fish up against the light, when in the male the posterior part of the 

 ventral region behind the intestines is seen to be translucent ; while in the female even 



