OF ARTS AND SCIENCES. 3 



first as a mere swelling of the median line, behind the hyoid bone ; this 

 (PL IV. figs. 3-5) grows quite rapidly ; the permanent fin-rays at 

 once make their appearance, — the anterior ones (the outer) first ; and 

 there is nothing special to note in the further development of the ven- 

 trals, which soon resemble, on a small scale, the ventrals of the adult. 

 The ventrals possess, at no time, embryonic fin-rays, like those of the 

 dorsal, anal, and caudal fins, formed from the longitudinal embryonic 

 fin-fold. In the pectorals, embryonic fin-rays also precede the forma- 

 tion of the permanent rays ; but in many bony fishes (PI. VI. fig. 5, 

 PI. X. fig. 1), these permanent rays appear very early, — before those 

 of other paired or unpaired fins, — the Crossopterygian stage being 

 passed while still in the egg. ■ 



A striking characteristic of the young of all bony fishes is the 

 extraordinary development of the pigment cells (chromatophores and 

 chromatoblasts), and the great changes they undergo during the 

 growth of the embryo. Pouchet* has more recently called attention 

 to the wide-spread existence of these pigment spots, so well known to 

 all students of Invertebrates. He studied them especially among the 

 Fishes, in connection with the atrophism of the color on the blind sides 

 of Flounders ; pointing most plainly to the partial atrophy of the 

 great sympathetic nerve, effected during the passage of the eye from 

 the right to the left, or vice versa, as the cause. The power of the 

 nervous system over the complicated system of pigment spots, which 

 produces eventually the coloring of the adult fish, is of course much 

 more readily traced in the younger stages, while the individual cells 

 are still isolated, and before their anastomoses have become so com- 

 plicated that it is well-nigh impossible, even in quite young specimens, 

 to follow the changes resulting from any special nervous excitement. 

 Conrpare, for instance, the simple chromatic system of cells of PI. 

 II. figs. 1-4, with the more and more complex anastomosing branches 

 of PL III. figs. 5 and PL IV. figs. 1-5. This is still better seen, perhaps, 

 if we compare PL VI. figs. 1-3 (young Flounders, just hatched from 

 the egg, and a couple of days old) with PL VI. figs. 5-7, showing the 

 gradual passage of the few, large, well-individualized chromatic cells of 

 PL VI. fig. 3, into the innumerable system of small cells, closely 

 packed and crowded in spots, so as to form the special design charac- 

 teristic of this species. f 



* Pouchet, G. Des Changements de Coloration sous l'lnfluence des Nerfs. 

 Archives de Physiologie et d'Anatomie. 1876. 



t Pouchet has succeeded in producing a white side in trouts, by destroying 

 the eye of that side. Rev. Scient. xiii. 1877. 



