46 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



the most noticeable portion of the tectum, especially in young animals. 

 The nuclei are closely crowded together, with a definite arrangement 

 due to the radially directed processes of the ependymal cells, which pass 

 through all the layers from the ventricle to the pia. Only one type of 

 cell body (Fig. 22, p) is evident, that being the small and rounded 

 form ; in Golgi preparations, it is slightly pear-shaped, and resembles 

 much the ependymal cell. But since the cells of this layer have pro- 

 cesses of a number of types, they cannot all be, as Fusari ('96) main- 

 tained, ependymal cells. They may fibrillate in any or all of the layers 

 outside the sixth. In Golgi preparations a very few spindle cells, like 

 those in layers 4 and 5, appear. Some of the peripheral cells (Fig. 22, a) 

 of this layer, as well as the very deep ones, may send to the surface a 

 process which ends in branching fibrillations beneath the pia. The 

 fibres from other cells were found to break up in layers 3, 4, and 5. 

 These fibres are often impregnated when none of their processes take the 

 silver, or vice versa. The cells next to adjacent layers, whether the 

 deeper or those nearer the periphery, are more likely to become impreg- 

 nated than those in the middle of the layer. 



The innermost layer (7), less dense than any of the preceding, is 

 composed of the bodies of the ependymal cells and the basal portions of 

 their processes. A reticulate portion of this layer (next to layer 6) is 

 not apparent in young specimens, and so I have not i*ecognized it as 

 a separate layer, but have included in layer 7 all that lies between the 

 gray layer (6) and the ventricle. 



In the adult brain there are scattered through this loose layer a few 

 large-bodied very irregular cells (Fig. 22, t), each having a multitude 

 of long beaded processes. I was unable to discover any neurite con- 

 nected with these cells. 



In order to simplify the diagram (Fig. 22\ I have omitted in all cases 

 the free fibrilLations. Inmost impregnations where there are any at all, 

 there are so many that only a few can be traced to any definite 

 medullated layer. Layer 3, however, certainly contains, among other 

 fibrillations, free branches from the optic layer (2). In layers 3 and 4 

 free fibrillations of fibres from cells in layer 5 are doubtful, because 

 any one of the many cells in the granular-layer (6) may have its fibre 

 impregnated though itself remaining clear.. 



Between the fillet layer (5) and the optic layer (2) there are two 

 especially dense fibrillar regions corresponding in gene/al to the two 

 bundles of dividing processes which arise from the cells of the nidulus 

 corticalis. 



