200 THE RADIATING OKGANS OF THE DEEP SEA FISHES. 



the canal (Fig. 56 cv). The outer stractureles.s sheatli of the canal is 

 separated from the periost of the bone on which it rests by a thin hiyer of 

 scattered pigment cells (Fig. 50 p). The radiating disc is situated in the 

 floor of the canal. Seen from the surface (Plate 11, Figs. 53-55) 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 hxyer, 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. 50, 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. 58-55). 



The cylinder cell plate (Figs. 56, 57 d) is perhaps not wholly composed 

 of cylinder cells equal in lengtli to the thickness of the plate. There is a 

 marginal zone (Fig. 50 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 i-eality 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 



