WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 45 



brain through the region of the nidulas corticalis into the longitudinal 

 fibre layer. Most of the cross-lying fibre bundles, which form the com- 

 missura mesencephali, lie below the longitudinal layer. Some of these 

 cross bundles seem to turn longitudinally after crossing the mid-line. 

 It may be that the uncrossed fibres of the fillet are a continuation of 

 these. The longitudinal fibres, at any rate, pass back in bundles to the 

 reo-ion of the anterior peduncles of the cerebellum. In any section 

 which cuts through the whole thickness of the tectum, whether cross or 

 parasagittal, some bundles will be shown (Plate 5, Fig. 25, Imn.). As 

 the tectum is dome-shaped, the more nearly median parasagittal 

 sections will cut the fibre bundles at the anterior and posterior ends of 

 the tectum, whereas the more lateral sections will show the fibres of the 

 middle of the tectum cut longitudinally. There is a rather distinct 

 portion of the fillet which arises from the anterior ventral part of the 

 tectum and, slanting upwards and inwards, passes through the nidulus- 

 corticalis region back towards the cerebellum, beneath and behind the 

 median boundary of the optic ventricles. The fillet fibres may be 

 roughly likened to the slightly curved fingei's of an open hand, palm 

 inward, wrist beneath the cerebellum, grasping the most of the gray 

 laj'er of the tectum. The gray of the posterior portion of the tectum 

 seems, however, to be outside the region surrounded by the fillet- fibre 

 bundles. 



The fibres of the commissura mesencephali cross just above the gray 

 layer in the anterior part of the tectum in the region of the torus longi- 

 tudinalis. According to Herrick they form a continuation of the series 

 found in the posterior commissure. 



Besides these fibres, there are in layer 5 a number of different forms 

 of cells : (a) Cells with rounded bodies (Plate 5, Fig. 22, ju) of the same 

 size as those (Fig. 22, p) in the next deeper layer (6) — the gray layer 

 — and with processes which may fibrillate into any one or all of the more 

 superficial layers (1-4) of the tectum, (b) Spindle-shaped cells (Fig. 22, 

 v) like those (t) characteristic of layer 4. When an axonic process can 

 be followed from the deep end of such a cell, it finds its way into the 

 fillet layer, but whether into the cross or longitudinal system I cannot 

 determine, (c) Long triangular cells (Fig. 22, o) with a single process 

 extending toward the periphery, and from each of the corners of the 

 deep end a process I'unning parallel to the fillet layer. (c?) Rounded 

 cells (Fig. 22, ir) with fibres which turn immediately into the fillet 

 layer and with very short dendritic processes. 



The next layer (6) is the gray molecular or granular layer. This is 



