32 THE PELAGIC STAGES OF YOUNG FISHES. 



accessory organs of hearing, and at the same time towards confirming the 

 serial homology of the ears with these organs, as maintained by Beard. 

 Leydig was the first to call attention to the similarity in structure between 

 these two classes of organs. 



The Sienna Flounder. 



Plates XIV. and XV. Figs. 1-14. 



Figs. 1 to 3 of Plate IX. in Agassiz's Young Stages of Osseous Fishes 

 (II. Development of the Flounders) belong to this species, which we may 

 designate as the Sienna Flounder. But the remaining stages figured on that 

 plate (Figs. 4-7) belong to another species, which we may call the Trans- 

 parent Flounder (Pseudorkomhts obbngus, Stein). The egg of the Transparent 

 Flounder is distinguished from that of the Sienna Flounder by having no 

 oil-globule, and no pigment on the yolk. In these respects, as well as in size 

 and general appearance, the egg agrees with that of the Tautog ; but the 

 young fish at the time of hatching is distinguished from the Tautog of 

 the same age hy its highly arched dorsal fin-fold, and by the position of 

 the vent, which is placed somewhat farther back. 



The average size of the egg of the Sienna Flounder is .05 mm., and its 

 oil-globule varies but little from .15 mm. From one to a dozen or more 

 are usually found daily during the summer months. The colorless chroma- 

 toblasts make their first appearance on the dorsal side of the embryo 

 and yolk-sac. at the time when the closing of the blastopore is wellnigh 

 completed (Fig. 1). Just after the closing (Fig. 2) we find that the chromato- 

 blasts are still uncolored, but more numerous, and a few of them present 

 from one to three short, fine pseudopodial elongations. The body of the 

 embryo shows a faint trace of diffuse yellow. 



Shortly alter the tail has begun to extend beyond the yolk, minute 

 round black dots begin to appear in some of the chromatoblasts. and these 

 become quite numerous by the time the stage of Fig. 3 is reached. At this 

 time, however, there is a considerable number of additional chromatoblasts 

 in which no black dot has appeared. The same is true of the stage of Fig. 4, 

 in which pale yellow chromatophores have become quite numerous, especially 

 on the tail. Fig. 5 shows a stage just before hatching, in which the devel- 

 opment of yellow pigment was not so far advanced as in the preceding 

 stage ( Fig. 1 ). 



