Reproduction and Growth 171 



at the end of the third or fourth year and at an average 

 length of ten inches. One of the biggest recorded in recent 

 years was fifteen inches long and six years old: it matures 

 therefore when it has lived one-half to two-thirds of its 

 possible life and reached about one-third of its possible 

 weight. The king salmon spawns generally in its fourth or 

 fifth year and dies immediately thereafter: it matures there- 

 fore at the end of its life and at its maximum weight. 



Once a fish has reached maturity its insides become prin- 

 cipally a factory for the production and storage of sex prod- 

 ucts. The basic elements are tissues called ovaries or testes, 

 depending on the sex, which produce either eggs or sperma- 

 tozoa. They form two masses, one each side of and above the 

 digestive organs. In the trouts and their relatives, the ovarian 

 walls do not entirely surround the eggs, and the ducts which 

 lead to the genital aperture are open troughs, so that eggs 

 can, and sometimes, especially in the process of stripping, do 

 escape into the body cavity. Such eggs can never be extruded, 

 and must be absorbed by the tissues. In most fish, however, 

 the ovaries are complete sacs with closed tubes leading to the 

 outside. This system seems more efficient than the mammals', 

 for in the latter there is a gap. The mammalian ovaries are 

 separate organs which have no connection with the uterus. 

 The ova are thus set free in the body cavity, and it is up to 

 slender arms of the uterus called Fallopian tubes to corral 

 them and bring them in. For this purpose the tubes have 

 funnel-like open ends located close to the ovaries, and a 

 vacuum-cleaner suction to sweep up loose objects, but even 

 at that an ovum sometimes lingers in the open spaces, and 

 once in a while is caught and fertilized in this embarrassing 

 position by a far-traveling sperm. "Extra-uterine" pregnancy 

 is the result. The primitive sharks have the same gap be- 

 tween the ovaries and the oviducts, and it looks as though 

 the closed system of the bony fishes is an improvement which 



