DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
661 
layers and vertical subdivisions. The dark concentric lines (c in fig. 18) are membra- 
nous portions, whilst the light intervening spaces (18 d), as well as the radii (18 e) pro- 
ceeding from the centre to each angle of the scale, are calcareous. In this preparation 
a little of the lower surface has been ground away, in order to bring the arrangement 
of the more internal parts into view. In fig. 19, the upper surface {a), the vertical 
pillars (5), the horizontal lamellae (c), as well as the oblique ones (c?), in which the 
two last series (6 and c) meet, are all calcareous. On the other hand, the central por- 
tion (e), the square lateral areolae (/'), and the inferior dark longitudinal lines (g) 
are permanently membranous. It will thus be seen that the scale consists of a com- 
bination of membranous and calcareous tissues, variously arranged. On examining 
these different portions still more minutely, beautiful structural details reveal them- 
selves ; some of these will be better understood after making a reference to fig. 20, 
which represents a still more highly magnified view of a portion of fig. 19. The 
uppermost tissue of the scale, or that to which the superficial tubercles seen in fig. 1/ 
belong, consists of very thin superimposed lamellae (fig. 20 a), traversed by numerous 
minute branching tubes, which open at the surface of the scale. The outermost of 
these lamellae are arranged parallel to the superficial undulating outline of the sec- 
tion, which circumstance, combined with the direction followed by the permeating 
tubes, leaves no doubt that the growth of these layers, like that of ordinary ganoin, 
has been the result of successive additions made to the upper surface of the scale. 
The structure thus produced is obviously one form of kosmine ; but beneath it 
there is another form which assumes a very different arrangement. Numerous large 
branching canals (fig. 20 h), fringed all round with minute tubuli. enter at the 
margin of the scale, and proceed towards its interior, traversing the concentric plates 
seen in fig. 18, nearly at right angles. Their general direction will be best under- 
stood on a reference to fig. 21, which represents one angle from the horizontal sec- 
tion of a scale, made in the plane of this series of canals. In the latter figure, they 
commence by open orifices (21 a) at each margin of the scale, and proceed at right 
angles to that margin, and in nearly parallel lines, towards its interior; sometimes 
they are simple throughout, whilst at others they unite with contiguous canals by 
means of anastomosing branches. Owing to their parallel direction, few of these 
reach the centre of the scale, but meeting similar canals coming from the opposite 
side, unite with them almost at right angles; whilst a single trunk (fig. 21 h), run- 
ning along the top of the radiating calcareous septum (fig. 18 e), combines all these 
various branches into a common system of canals, which furnish the upper tissues of 
the scale with a portion of their nutrient fluids. Sometimes these canals send small 
branches upwards (fig. 20 c), which open at the exterior of the scale, whilst others, 
proceeding in the opposite direction, open at its inferior surface. At fig. 2 1 c, the 
minute kosmine tubes given off by these canals have been omitted from the drawing, 
in order to show the undulatory lamellse or lines of growth, which run parallel with 
the external surface, as we also saw to be the case in the vertical section, fig. 20 a. 
MDCCCLI. 4 Q 
