HIGH SCHOOL ZOOLOGY. 49 



Little more need be said about the habits of the catfish. It 

 is remarkable for its tenacity of life, is regarded as a fair food 

 fish, and has accordingly received some attention from pisci- 

 culturists who have found that it prospers also in ponds and 

 streams of other regions besides that in which it naturally 

 occurs. 



66. The artifical hatching of fish-spawn with the object of stocking 

 depleted waters and increasing the food supply is now being carried 

 on very vigorously in Canada and the United States, as well as in some 

 European countries. In Ontario the chief hatcheries are at Newcastle 

 and Sandwich, whence vast numbers of the fry of White Fish, Lake 

 Salmon, Pickerel, etc., are distributed for replenishing the waters of 

 the Provinces. The eggs are hatched out in troughs supplied with con- 

 stantly renewed water at a certain temperature, and thus many of the 

 causes which, under ordinary circumstances m.iy lead to the arrest of 

 the development of the eggs are obviated. Some notions as to the 

 gradual development of the body in a catfish may be gathered from 

 Fig. 19. 



'&■ 



The egg while still within the ovary of the mother, (1) is about one- 

 eighth of an inch in diameter ; it has two coats, the outer of which is 

 penetrated by minute canals through which the necessary nutriment for 

 the growth of the egg passes inwards. When the egg is laid, the space 

 between the two coats increases in size, and the two constituents of the 

 yolk (the formative yolk which gives rise directly to the body of the 

 embryo and the food-yolk which is utilised as food by the embryo), 

 formerly evenly distributed, now tend to accumulate at opposite poles (2). 

 The formative yolk with its contained nucleus begins to segment, the 

 result being a disc of small cells lying upon the surface of the food-yolk. 

 These cells gi'adually extend over the whole of the egg, those at the 

 pole arranging themselves into the three layers of the embryo, which 

 already, during the second day assumes a fish-like form (.S). It is from the 

 three embryonic layers (epiljlast, mesoblast and hypoblast) that all the 

 organs of the body are developed (4) ; a similar arrangement of these 

 exists in all vertebrate animals. The embryo does not escape from the 

 egg membranes until the sixth day, when, although only one-third of an 

 inch in length (5), development has advanced to a considerable extent. 

 Thus the heart is seen in front of the yolk-sac, from the vessels of 

 which it collects the blood enriched by contact with the yolk, and pro- 



