172 The Life Story of the Fish 



they invented after they had branched off from the main line 

 of evolution. 



The amount of space taken up by the ripe sex products 

 in most fish is comparatively tremendous. The ovaries of a 

 cow form only an infinitesimal fraction of her total weight, 

 whereas the ovaries of a salmon may make up from one- 

 fourth to one-fifth of her total weight. In most fish the ripe 

 eggs not only completely fill the body cavity but swell it out 

 to a very matronly plumpness. In some species the whole 

 mass of eggs comes to the state of ripeness at approximately 

 the same time, and is released at one time, or at most over 

 a period of a few days, immediately after which the process 

 of forming new reproductive cells begins again. No sooner 

 is one lot disposed of than another lot is on its way, so that 

 the mature female has inside her eggs at some stage of 

 fruition at any time except immediately after spawning. In 

 other species not even this hiatus occurs, for the female car- 

 ries several generations of eggs at once. Ripe eggs to be 

 spawned in the immediate future, unripe eggs to be spawned 

 in the next cycle, and the tiny rudiments of eggs to be 

 spawned in years to come, are all present together. 



The interval between spawning varies with the species. In 

 some of the small tropicals it is astonishingly short: four 

 days after the production of one batch of eggs the female is 

 ready with another, while the male is apparently never 

 unready to do his part. This rate cannot continue indefinitely. 

 Longer lapses, and rest periods, occur. But the number of 

 times which they are capable of spawning during their lives 

 seems limitless. 



The temperate-zone fishes proceed at a more deliberate 

 pace. Most of them have definite breeding-seasons, and 

 spawn but once a year. The number of spawnings therefore 

 depends on the length of life of the fish. In the trouts it is 

 limited by this factor to a maximum of seven or eight, with 



