CLASS PISCES 



361 



all of the actions of fish are dictated by instinct, though they also form 

 habits. They possess little intelligence. 



387. Reproduction. — The sexes of fish are separate. Fertilization 

 may take place by the male depositing the seminal fluid, or milt, which 

 contains the sperm cells, over the eggs, called roe, at the time of laying; 



Pro-f-ophsnj 

 anc/ nac/eus 



B/asfon^eres 



B/asfoc/er/rr 



B 



f^eural groove 



Embryo 



Embryo 



Head ^ ^'^''^ ^'^^ 



Fig. 255. — Stages in the development of a salmon. A, the egg before development 

 begins. B, four-celled stage. C, the blastoderm. D, blastoderm elongated in the direc- 

 tion of the longitudinal axis. E, the blastoderm with thickened margin, and the beginning 

 of the neural groove. F, the yolk nearly covered by the growth of the margin of the 

 blastoderm and the embryo elongating. G, the embryo raised up on the top of the egg, 

 the yolk now being enclosed by the yolk sac. H, the animal much older, having developed 

 the external appearance of a fish, but with the yolk sac still attached below. {Mostly from 

 Parker and Haswell, " Text-book of Zoology " after Henneguy, by the courtesy of The Mac- 

 millan Company.) 



or it may occur a short time after the milt and roe are passed out into the 

 water. The female is usually the larger sex, in some cases her length 

 exceeding that of the male several times. Most of the eggs of fishes are 

 relatively small and surrounded by a protective covering which may be 

 adhesive and by which they may be attached to each other and to solid 

 objects. They may be laid separately, or they may be deposited in 

 groups. If laid separately, as in the case of some marine types, very 

 large numbers of eggs are devoured by other fish and by other animals. 



