THE VERTEBRATES 



443 



In all species of vertebrates there are both male and female 

 individuals ; and new individuals always originate from 

 egg-cells fertilized by 

 sperm-cells. Asexual 

 reproduction and par- 

 thenogenesis are not 

 known among these 

 higher animals. 



The eggs of most 

 fishes are laid in shal- 

 low water in quiet 

 places, and sperm-cells 

 discharged into the 

 water fertilize the eggs. 

 The enormous loss of 

 eggs and young fishes 

 under natural condi- 

 tions has led to artifi- 

 cial fish-culture under 

 the control of various 

 states and the United 

 States Bureau of Fish- 

 eries. The methods of 

 hatching are a scien- 

 tific application of na- 

 ture 'sway. Fishes are 

 caught by nets at the 

 breeding season, and 

 pressure on their ven- 

 tral surfaces causes ex- 

 trusion of eggs and 

 sperm-cells. These are 

 mixed in water and 



allowed to stand in shallow pans or water-tight boxes until 

 the sperm-cells have by swimming reached and penetrated 



FIG. 149. Stages in development of a fish. 

 bl, germ-disc, remainder of the egg is yolk ; 

 emb, developing embryo ; ys, yolk-sac at- 

 tached to ventral surface after hatching. 

 (From Parker and Haswell.) 



