5 NAKAGAIVA. [Vol. IV. 



shaped and proximal to the mesocoele ; are in single row pressed 

 side by side. Posteriorly the cells are more numerous, and in 

 the anterior and ventral portions they expand to form a wide 

 band, as has been mentioned in reference to the layer D. 



[G.] In this layer fibres or striations are seen connecting the 

 ependyraa below with the layer F above. There are also some 

 fibres running around the ependyma layer {H, Fig. 6). 



2. Homologies in the Corpora Bigemina of the Saurop- 

 siDA. — Although in reptiles and birds the structure of the optic 

 lobes is not so clearly marked as it is in the frog, yet the homology 

 of the layers in these forms is not difficult to trace. Beginning 

 with the layer A, we find it constant in Tropidonotus, Eviys, and 

 Colnmba (Figs. 7, 8, 9, 10, A). These fibre-layers are trace- 

 able to the optic nerve, as is the case in Rana. They are, how- 

 ever, much reduced in thickness, especially in Coluviba. In 

 Tropidonohis (and in most places in Cohimba also), this layer 

 is not well differentiated from the layer B, which, in turn, is not 

 marked off from the layers below (Fig. 7). In the turtle it is 

 slightly better marked ; but, owing to the presence of cells in 

 the next layer, it is not so clearly brought out as in Rana. In 

 Columba, layer B is distinct at the inner corner of the tectum 

 opticum from which Fig. 9 has been taken. Layers B, C, and 

 D are not differentiated in TropidonoUis, while the latter two 

 {C and D) are not distinguishable in the turtle and pigeon. 

 The layer CD consists of fibres and cells, representing the two 

 distinct layers in the frog. The cells are less spindle-shaped 

 than in the frog, although the fact that their processes run 

 mainly upwards and downwards (or strictly speaking, radially) 

 makes them appear rather elongated in the same direction. In 

 TropidonoUis cells are irregularly scattered about, but in Emys 

 they are more numerous toward the mesocoele ; the rows of 

 cells bordering the layer E being somewhat larger than the 

 rest (Fig. 8, CD ; Fig. 7, BCD). Colnmba exhibits a high degree 

 of differentiation toward the inner edge of the tectum opticum. 

 This layer is divided up into three sublayers of cells, with fibre- 

 lamina intervening between them (Fig. 9, CD). The character 

 of the cells is similar to what is found in preceding forms. 



E, or the layer of the commissura tecti optici, is conspicuous 

 in all the specimens. It is thicker in TropidonoUis than in 

 either Emys or Rana. TropidonoUis also presents a singular 



