180 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 —eg. 
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. 189-191, which have 
been reduced about two-thirds. From the figures the — 
character of the egg 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). 
