796 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
hatching, while in the gurnard a third-day embryo shows irregular patches of yellow pig- 
ment, with which black spots are also mingled (Pl. XVI. fig. 8). The coloration in other 
species will be noticed on a following page. 
In transverse section this fin-membrane (ef) consists merely of a simple median fold 
of the double-layered epiblast—the outer flattened corneous layer, and the inner sensory 
layer, which proceeds into the narrow fissure separating the two lamellae of the fin 
(Pl. VIL. figs. 3,6). This fissure enlarges close to the trunk, and is continuous with a 
spacious subepidermal chamber which extends all round the latter, and is well seen in 
late larval stages in section (PI. VII. fig. 6) and surface view (PI. XVI. figs. 1, 3). A jelly- 
like lymph fills up this cavity, which, as already pointed out, becomes extraordinarily 
enlarged in the cephalic region. All along the trunk such a space exists in a modified 
degree, and delicate nerve-strands pass across it from the spinal cord to the sensory 
papille in the skin. Along the tail the interspace is narrowest (ss, Pl. XI. figs. 15, 17), 
but on the ventral side, as the root of the tail is approached, it enlarges and forms a 
spacious fissure in the anal region (ss, Pl. XI. fig. 14). It is in this chamber, limited 
on each side by the epiblastic fin-fold, that the rectum (ig) pushes its way, and before 
the anus is formed sends out a strand of loose cells, extending from the base of the 
urinary vesicle to a point midway down the expanse of the fin-membrane. 
The hind gut, as already indicated, ends blindly, and does so for a period varying 
very much according to the species. The anal column of cells, before and after a 
lumen is formed, passes down the centre of the fissure (ss), and is apparently held in 
place by the tenacious plasma (, Pl. VII. figs. 12, 13), in which granules subsequently 
appear, and forms a matrix surrounding this part of the intestinal tract. As formerly 
mentioned, the anus does not extend to the ventral margin of the fin, but opens at the 
side about midway (a, Pl. VII. figs. 14, 15). In this continuous embryonic fold the 
permanent unpaired fins of the adult fish are formed—arising, as Barour said, by local 
hypertrophy (No. 11, p. 78), though no less by atrophy of the parts between the 
ultimate fins. LEREBOULLET refers to this atrophy in Perca, when he says the margin 
becomes indented where the three vertical fins in that species will finally remain 
(No. 98, p. 634). These local indentations mark the atrophy of parts of the embryonic 
membrane, which finally disappear, leaving the prominent and strengthened remnants 
of the once continuous fin to form the permanent unpaired fins. Before this atrophy 
of the transient portions and the hypertrophy of the permanents parts, the sites of the 
ultimate fins often appear to be indicated by remarkable aggregations of pigment. Thus, 
in the advanced embryo of Plewronectes flesus, a striking development of pigment- 
corpuscles takes place in the dorsal and ventral portions of the embryonic fin. Scattered 
pigment occurs along its whole extent behind the pectoral region, though it is sparse ; 

but certain parts in an early stage are distinguished by more abundant coloration, and 
in the thirteenth-day flounder, referred to, a patch of brownish-yellow pigment-spots, 
arranged in a radiate manner, is seen with black spots intermingled (Pl. XVI. fig. 1), as 
also in the undetermined Pleuronectid figured on Pl. XVIII. fig. 1, and in Agonus on the 
