364 H. H. SWINNERTON. 
DESCRIPTIVE. 
The Development of the Pectoral Skeleton of 
Salmo salar. 
Younger stages than any at my disposal were described by 
Ducret. From his observations it is seen that the cartila- 
ginous skeleton first begins to appear as procartilage in 
embryos 9°6 mm. long. When they are 13°4 mm. long the 
skeleton has become cartilaginous, and consists of a lamina 
lying in the fin, and of a basal plate in the body wall. 
Stage I (figs. 1 and 2).—At this stage the fin is a half- 
moon-shaped fold which, on account of the relative smallness 
of the embryo as compared with the yolk-sac, points dorsally. 
Consequently the morphologically ventral side of the fin faces 
laterally. Itis largely supported internally by a thin concavo- 
convex plate of hyaline cartilage (f. p.) with its concave face 
towards the inner side. Its shape calls to mind that of the 
larval Poly pterus (Budgett), and it differs chiefly in the 
breadth of its base of attachment. The fin-plate is continuous 
with the girdle (sc. and co.), which lies in the body wall, and 
is also hyaline. ‘These two regions are far from being sharply 
marked off from one another either by histological structure 
or difference of orientation. ‘he scapulo-coracoid region is 
but a continuation of the fin-plate, and with that forms the 
saine concayo-convexity. Such figures as that of Ducret’s 
(pl. Il, fig. 12) give the impression that the fin-plate is 
attached at right angles to the plane of the girdle or “ plaque 
vasale,”’ and therefore that the portion of the girdle dorsal to 
the line of attachment is the scapula, and that which is ventral 
is the coracoid. Such an impression is erroneous and is due 
to the study of sections without the aid of reconstructions. 
The boundary between these two regions can therefore be 
ascertained only by following the line of attachment of the 
fin-fold to the body. ‘his is represented by a dotted line 
(g. b.) in the figures, and may be spoken of as the glenoid 
line. It is along this that the glenoid articulation is formed 
