No. I.] OPTIC LOBE LAYERS IN LOWER VERTEBRATES. 7 



arrangement of E. This fibre-layer in its lower half is crowded 

 by cells which apparently have extended outward from the layer 

 below. These intruding cells are lenticular in shape, with their 

 longest diameter arranged radially (or vertically in Fig. 7). In 

 Coluniba it is subdivided into two strata, — an outer stratum of 

 horizontal fibres (of the commissure), and an inner stratum of 

 vertical fibres. These strata are further distinguished by the 

 presence of cells different in each. But as I am inclined to con- 

 sider these cells as properly belonging to the layer F, they will 

 be described later on. 



In TropidonoUis the layer F is well provided with large multi- 

 polar cells, with nucleoli {mesencephalic nucletis of the Trigejninns), 

 and the ordinary cells are much more abundant than in Rana, 

 but are not packed side by side, as we have seen them in the 

 latter. Besides, there are, as has been mentioned, in connection 

 with E, a number of lenticular cells proceeding upward from 

 F (Fig. 7, F). Although in Eviys the large cells are not nearly 

 so numerous as in Tropidoiwtiis (being mainly confined along 

 the junction of the lobes), their scarcity is compensated, as it 

 were, by the elaborate stratification observed in this layer. In 

 most places there are five strata divided by fibre-laminae. These 

 strata are composed of ordinary cells which are somewhat spindle- 

 shaped. Returning now to those cells found in the layer E of 

 Colwnba, which form the first two sublayers of F, we observe, 

 first, in the horizontal stratum, or among the fibres of the Com- 

 missura tecti opticis, that the large multipolar ganglion cells are 

 elongated in the direction of the fibre-tract ; but the smaller cells 

 do not present any definite direction. Secondly, in the vertical 

 fibre-stratum, ganghon cells do not present any definite direction, 

 while on the other hand the lenticular cells run vertically ; that 

 is, in the trend of fibres. The third stratum, or the layer F 

 proper, consists of ganglion and small spherical cells with nucleoli 

 and with processes extending in all directions. 



The layer G has no marked feature, except in Columba, where 

 it presents more of the appearance of molecular structure than 

 distinctly fibrillar. The ependyma is thickest in Rana, consist- 

 ing of from five to six rows, while in TropidonotJis and Columba 

 it is very thin (Figs. 6, 7, 8, 9, H). 



