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, PI. V. figs. 4, 6, 9 ; PI. XIX. fig. 7), 

 the fins change to a more vertical situation (pf, PL XIII. figs. 1, 6 ; PI. 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 

 lie, 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 Wolffian 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 mesob]ast assumes a columnar character, and is seen as a transversely 

 striated central bar in cross-section (x, PL VII. fig. 2). This plate (x) 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 VII. 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 (PL 

 XII. fig. l) being now exchanged for an oblique vertical attachment (PL XIII. fig. 

 1 ; PL XVI. 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; PI. XV. fig. 2; PL XVIII. figs. 2, 10, 11). Meanwhile a pectoral bar appears 



