a 
200 THE RADIATING ORGANS OF THE DEEP SEA FISHES. 
the canal (Fig. 56 cv). The outer structureless sheath of the canal is 
separated from the periost of the bone on which it rests by a thin layer of 
scattered pigment cells (Fig. 56 p). The radiating disc is situated in the 
floor of the canal. Seen from the surface (Plate 11, Figs. 58-55) it has the 
shape of a rhomboidal spindle with obtuse points. The most important part 
of it is a single layer of slender somewhat irregularly cylindrical cells (Plate 
11, Figs. 56, 57 d) which form an epithelial layer. A stout main disc nerve 
(Plate 11, Figs. 53-56 n) and one or two smaller accessory nerves (Figs. 53- 
55 n’) lead to this cylinder cell layer. The main nerve rises obliquely 
towards the centre of the cylinder cell layer, arrived at the base of which it 
suddenly divides into numerous radiating branches extending paratangen- 
tially below the cylinder cell plate. Here also an exceedingly dense capillary 
network (Figs. 56, 57 b) is met with forming a conspicuous blood-vessel 
layer between and under the basal parts of the cylinder cells. The large 
vessels leading to this network (Figs. 53-56 b’) are very conspicuous, particu- 
larly in surface views (Figs. 53-55). 
The cylinder cell plate (Figs. 56, 57 d) is perhaps not wholly composed 
of cylinder cells equal in length to the thickness of the plate. There is a 
marginal zone (Fig. 56 m), where in transverse sections through the plate 
the cells appear to be polyedrical and to form several layers. It is probable, 
however, that also here the cells are in reality cylindrical and form a single 
layer and that the appearance seen is only the consequence of the marginal 
cells not extending, as the others do, in the plane of the section. They 
would therefore not, as the cells of other parts of the plate, be cut longi- 
tudinally, but more or less transversely, and thus assume the appearance of 
polyedrical elements lying in several layers. Also in the centre of the plate 
there seems to occur a slight irregularity. 
The greater part of the elongated rhomboidical plate formed by this 
cell layer is flat. Its margins, in which the apparent multiplication of cell 
layers mentioned occurs, are markedly raised. In the middle, along the 
short diagonal of the elongate plate, a transverse groove extends. All 
the cylinder cells with the exception of the marginal ones, arise vertically 
from the floor of the plate. Most of them show a distinct curvature increas- 
ing towards the upper end (Fig. 57). In the central groove the cylinder 
cells lie close together, in the other parts of the plate they are apparently 
separated by intercellular spaces of considerable width (Fig. 57). Possibly 
this appearance is merely the expression of the presence of thick lateral cell 
