802 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
supported most clearly by the development and early condition of these structures in 
Teleosteans. 
From the primary horizontal position (pf, Pl. V. figs. 4, 6, 9; Pl. XIX. fig. 7), 
the fins change to a more vertical situation (pf, Pl. XIII. figs. 1, 6; Pl. XVI. figs. 
6, 8), though still connected by a lengthy attachment to the side of the embryo. The 
mesoblast of the fin-plate may be traced to a mass of cells in which the Wolffian ducts 
Jie, and out of which they are developed (Pl. VII. figs. 1, 2). If these ducts, as appears 
to be the case, arise as lateral ridges or diverticula of the somatopleure, then the meso- 
blastic cells of the fins must be pronounced somatopleuric. But no ridge of somatopleuric 
cells, comparable to the Wolffian ridge of higher forms, has been recognised in fishes, and 
we must regard this mesoblast as indifferent, and forming an “intermediate cell-mass” 
adjacent to the excretory system. The proximity of the Wolftian duct and the base of 
the pectoral fin is very noticeable (Pl. VII. fig. 7). The fins gradually become dis- 
connected from the blastodermic yolk-sac, and about the time that they are free a median 
stratum in their mesoblast assumes a columnar character, and is seen as a transversely 
striated central bar in cross-section (x, P]. VII. fig. 2). This plate (@) is gradually con- 
verted into cartilage, and extends from the base of the fin, where it is thickened almost to 
the distal border, at which it thins out and ceases (Pl. VII. figs. 1-3). Around this fan-like 
cartilaginous plate the adjacent mesoblast develops rapidly, especially near the proximal 
attachment to the trunk, so that a stout peduncle is formed (Pl. VII. figs. 1, 2). 
Viewed from above, in the living embryo, the fin appears as in Pl. VII. fig. 10, the outer 
and anterior margin presenting many protoplasmic processes, which seem to bind it to 
the epiblast over the yolk. The pigment-corpuscles, moreover, may be regularly disposed 
on the fin. Each fin, therefore, consists of a thickened stalk and an outspread distal 
expansion (pf, Pl. XII. fig. 6a), traversed from the base almost to the summit by a 
flattened plate of cartilage which is imbedded in a mass of indifferent mesoblastic cells, 
destined to become the muscles of the limb, and forming the main mass of the peduncle 
(Pl. VIL. fig. 7). The basal part continues to become thicker, and later is disproportion- 
ately enlarged, while at the same time the more distal parts expand like a fan, and 
become thinner and more transparent, save where the delicate radial striations pass. 
The part towards the distal border in many forms quickly exhibits pigmentation, e.g., in 
T. gurnardus (Pl. XII. fig. 1), Molva vulgaris, Cottus, and Liparis, radially disposed 
yellow and black pigment-spots being intermingled in the distal parts of the fin in the 
first-named species (Pl. X. figs. 2,3; Pl. XVI. fig. 8), or again, rich orange stripes in 
Liparis (Pl. XVI. fig. 7). 
During the third week after hatching the “rotation” of the fin has reached a stage 
at which its position is seen to be wholly altered, the original horizontal position (PI. 
XII. fig. 1) being now exchanged for an oblique vertical attachment (Pl. XIII. fig. 
1; Pl. XVL. figs. 3, 4, 7). The rotation continues until its basal attachment is almost 
perfectly dorso-ventral, and therefore at right angles to its primary position (Pl. X. figs. 
2, 3; Pl. XV. fig. 2; Pl. XVIII. figs. 2,10, 11). Meanwhile a pectoral bar appears 
