XI 



PEACTICAL HINTS— FISHES 



565 



numbers in the tow-net 

 but these are not so con- 

 venient for investigation 

 on account of their re- 

 duced size. As there is 

 little doubt that the Tele- 

 ostei have been evolved 

 out of ancestral forms 

 with large eggs investiga- 

 tions are particularly de- 

 sirable on those teleosts, 

 mostly freshwater forms 

 inhabiting warm climates, 

 in which the large size of 

 the egg has been retained. 

 There is an important 

 field for investigation in 

 the embryology of tropi- 

 cal freshwater fishes. Of 

 individual families the 

 Siluridae, Characinidae 

 and G-ymnotidae call 

 especially for investi- 

 gation. 



Dipnoi. — The Lung- 

 fishes form a group of 

 much importance to the 

 Vertebrate morphologist 

 on account of, on the one 

 hand, their great an- 

 tiquity and the retention 

 of many archaic features 

 in their organization and, 

 on the other hand, of the 

 fact that they present to 

 us foreshadowings of vari- 

 ous features which become 

 prominent characteristics 

 in ' the tetrapoda or 

 terrestrial animals. A 

 knowledge of their em- 

 bryology consequently 

 became one of the great 

 desiderata of Vertebrate 

 Embryology. The first 

 discovered of the three 

 surviving representatives 

 of the group — Lepido- 





rh 



ol 



Pf 



D 



Fig. 250. — Blastoderms and embryos of Trout 

 (Salmo fario). (After Kopsch, 1898. ) 



E, eye ; ot, otoeyst ; p.f, pectoral fin ; rh, rhombencephalon ; 

 y, exposed surface of yolk. 



