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 

 (PI. VII. 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 (PL VII. fig. 6) and surface view (PL 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 

 papillae in the skin. Along the tail the interspace is narrowest (ss, PL XL 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 XL fig. 14). It is in this chamber, limited 

 on each side by the epiblastic fin-fold, that the rectum (hg) 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 (x, 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 Balfour 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. 93, 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 Pleuronectes jiesus, 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 



