WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES, 45 
brain through the region of the nidulus 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 
region 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 cerebeflum, beneath and behind the 
median boundary of the optic ventricles. ‘The fillet fibres may be 
roughly likened to the slightly curved fingers of an open hand, palm 
inward, wrist beneath the cerebellum, grasping the most of the gray 
layer 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, ) 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. (6) Spindle-shaped cells (Fig. 22, 
v) like those (c) 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, 0) with a single process 
extending toward the periphery, and from each of the corners of the 
deep end a process running parallel to the fillet layer. (d) Rounded 
cells (Fig. 22, 7) 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 
