THE SIENNA FLOUNDER. 33 



At the time of hatching, the small round black dots (.005 mm.) are few 

 in number, and nearly evenly distributed over the yolk-sac and the entire 

 body and head. In front of the end of the tail a few of these dots arc seen 

 on the embryonic fin-fold, both above and below the muscular axis. The 

 yellow chromatophores are rather more numerous, and from two to three 

 times as large (.01 mm. or more) as the black dots. The yolk-sac shows 

 only a few of these yellow cells along each side of the body, and none on 

 the ventral half. The globule has one or two yellow dots to ten or more 

 black ones. The yellow chromatophores are evenly distributed over the 

 head and trunk, but stop short in front of the end of the tail. On the dorsal 

 fold they form a single row along the margin of the middle third; half- 

 way between the vent and tip they form a loose cluster, both above and 

 below the axis of the tail. Only two or three are to be seen on the ventral 

 fold between the vent and the loose cluster. After hatching, the color varies 

 considerably from hour to hour, according to the contracted or expanded 

 state of the chromatophores. Fig. 6 (12 hours) shows both the yellow and 

 the black cells in a state of maximum expansion. The yellow pigment of 

 the dorsal fold appears in the form of brownish yellow blotches, and the 

 whole body has a diffuse bright yellow hue. The black pigment-cells have a 

 rather coarse dendritic form, and the caudal cluster on the ventral side has 

 assumed a linear arrangement. 



The dimensions of the young fish of twenty-four hours are as follows: — 



Length 3.10 in in. 



Yolk-sac 1.00 X .35 " 



Yolk sac t.. vent 20 " 



Vent to tip 1.55 " 



Height (at middle of yolk-sac) 55 " 



Width of head 325 " 



In specimens of this age the black chromatophores are more or less 

 angular or branched, but much smaller than in the specimen of twelve hours 

 (Fig. G). They usually preserve the uniform distribution seen at the time 

 of hatching. The yellow cells, on the contrary, exhibit a remarkable power 

 of expansion, especially on the body ; and on the embryonic fold they con- 

 centrate into irregular, more or less brownish patches, which vary much in 

 size, shape, and number. Their more typical arrangement is that seen in 

 the stage of Fig. 8 (60 hours). Three blotches hold a marginal position 

 near the middle of the dorsal fold ; four are placed about half-way from the 

 vent to the tip. two above and two below ; and two more, one dorsal and 



