No. I.] OPTIC LOBE LAYERS IN LOWER VERTEBRATES. r 



consists chiefly of fibres which are traceable to the optic nerve. 

 Some of these fibres sweep over the tectum opticum and pass 

 into the Thalamencephalon (and ultimately into the cerebrum). 

 [-5".] This is the first gray layer, consisting of loosely arranged 

 cells. These cells are of the spherical description, and the 

 majority of the polar processes point toward the mesocoele. 

 [r.] The second white layer differs from the first in two respects : 

 it contains vertical fibres descending from the layer B, and 

 secondly, none of its fibres seem to pass toward the hemispheres. 

 The layer [Z>] is a compact cell-layer. These cells are somewhat 

 spindle-shaped, the processes running radially. They are ar- 

 ranged in from seven to nine concentric rows, and at places one 

 or more of these rows are seen separated from the rest. Ante- 

 riorly, in front of the mesocoele, this layer forms a wide band owing 

 to the interspaces of fibres formed between each cell-row, and 

 along the proximal side of the lobe it is pushed apart to admit 

 the exit of the Trigeminal nerve-tract, which proceeds from the 

 layer F, as will be described further on (Fig. lo), and also for 

 the fibres of the coimnissura tecti optici. [^5'.] The fibres descend- 

 ing from the layer D to F, the commissural fibres apparently 

 connecting the layers D and F ot the opposite sides, and prob- 

 ably fibres coming from the cerebrum constitute the afferent por- 

 tion of the third white layer (Fig. 6, E) (Fig. lo). The efferent 

 portion is composed of fibre-tracts which, according to my obser- 

 vation, partly contribute to the III, IV, and V (probably the 

 VI also) nerves, starting from the layer F. The tract con- 

 tributing to the third nerve arises from the ventral portion 

 (below the mesocoele), that of the fifth nerve from the proximal 

 side, and those of the fourth and sixth from the dorso-posterior 

 portion. The tract of the VI probably passes through the 

 "nucleus magnus " ; but my observation as to the course of 

 this tract has been very incomplete. [F.] The layer /^ is marked 

 by the presence of large multipolar cells, constituting the well- 

 known mesencephalic nucleus of the Trigeminus. They each 

 have a nucleolus, and are situated mainly in the anterior and 

 proximal portions, where the fibres of the III and V nerve- 

 tracts, respectively, are very distinct (Figs. lo and ii).^ The 

 greater part of the cells composing this layer are rather spindle- 



2 The multipolar cells are drawn rather too large in these figures. The real pro- 

 portion is represented in Fig. 6. 



