436 





extracted from the yolk-sac, the latter is absorbed, and the young fish begins to 

 feed for itself. At the end of three months the adult form is attained, the fish 

 being then hardly an inch in length. 



The length of time which the hatching process takes, and the rapidity with 

 which the developmental process runs vary much in different species of Tehosts, 

 -and is largely dependent on the temperature of the water, spawn deposited in 



Fig. 8.— Diagram of skvekal isTauks in DKVKLOPirENT of Catfish. (Modified from Ryder). 

 1, ovarian esrg ; 2, egg in which formative yoke has separated to upper pole ; 3, embryo of second day ; 

 4, section through such an embryo, showing »'pil)last with nervous system above, hypoblast below, and 

 between them the mesoblast and the notochord ; 5 embryo of sixth day. 



the cold water of October and November not liatching till the following spring 

 in contrast with the rapid process depicted above, which occurs in the early sum- 

 mer. The great ditterence in size which is to be observed between the eggs of 

 different species is not merely pi'oportional to the size of the species, although, as 

 we shall find, large varieties of bro.)k trout lay larger eggs than small varieties ; it 

 is largely a question of the relative am'»unt of food-yolk provided for the embryo, 

 ^nd niay be therefore also attributed tn the hatching habit of the species, the 

 longer or shorter time which the embryo takes to burst the egg-shell and to begin 

 to feed for itscU". 



The following table gives the number of eggs that have been counted to 

 ■a quart in different species: — 



Mackerel 1,267,728 



Pickerel (Stlzosiedium vitreum) 120,000 



Whitefish 36,000 



Striped Bass 24,363 



Brook Trout 11,000 



Lake Trout 8,720 



Atlantic Salmon 4,272 



Winninish j , 3.h00 



