l8o DEVELOPMENT OF FISHES 



of the closely related Ganoid and Teleost, than in those of 

 a Reptile and a Bird ; and even among the members of the 

 single group, Teleosts, there are more striking embryolog- 

 ical differences than those between Reptiles and Mammals. 

 Adaptive characters have entered so largely into the plan 

 of the development of fishes that they obscure many of 

 the features which might otherwise be made of value for 

 comparison. And until the controversies regarding some 

 of the most fundamental principles in embryology — e.g. 

 the importance of the loss or gain of food yolk — shall be 

 decided, it seems impracticable to use the plan of develop- 

 ment as in any strict sense a guide in phylogeny. 



It is, accordingly, rather with the view of contrast- 

 ing the groups of fishes, whose external features have 

 hitherto been compared, that the present chapter seems 

 of especial importance. They may briefly be reviewed in 

 their (A) spawning habits, (B) the mode of fertilization 

 of their eggs, (C) their embryonic, and (D) larval de- 

 velopment. 



A. EGGS AND BREEDING HABITS 



The eggs of typical fishes in Figs. 186-199, illustrate 

 how wide a range occurs in their shapes and sizes. All 

 are of about actual size, except Figs. 1 89-191, which have 

 been reduced about two-thirds. From the figures the 

 character of the &^g membranes may also be contrasted. 



Among Cyclostomes, which are usually looked upon 

 as of close genetic kinship, there appears a striking dif- 

 ference in the characters of the eggs. Those of Bdello- 

 stoma and Myxine (Figs. 186, 187) are large and bluntly 

 spindle-shaped, encased in a horn-like capsule ; those, on 

 the other hand, of Petromyzon are minute, spherical, and 

 enclosed in delicate and jelly-like membranes (Fig. 188). 



