50 Journal of Comparative Neurology. 



layer consists of obliquely directed fibres, as in the second layer. 

 T\\Q fifth /ajrr consists of transverse fibres derived from the crura 

 lobi optici. This layer is clearer in color than the remainder of 

 the tectum, owing to the absence of granular matter, and also to 

 the fibres not becoming so readily colored as the other parts of the 

 tectm ; some radial fibres run across this stratum also. The sixih 

 layer consists of small cells arranged on branched stems which are 

 prolonged into the radial fibres occurring in the other layers. 

 The seifcnfh is a layer of connective tissue of varying thick- 

 ness in different parts of the tectum ; it forms on its external 

 edge a support for the cells of the sixth layer, and internally ter- 

 minates by a single layer of epithelial cells, which forms a 

 smooth surface towards the ventricle of the optic lobe ; this 

 ependyma forms a support for the radiating fibres of the crura 

 lobi optici in their passage from the torus semicircularis to the 

 tectum ; this connective tissue is composed of an inextricable 

 network of fibrillse derived from the epithelial layer of cells. 



" The cells of the sixth layer are of two different forms; 

 those situated on the inner edge, which make the terminal en- 

 largement of the radial fibres, and those of the deeper part of 

 the stratum, which are atlached to the sides of the fibres of the 

 first, like grapes to their stalk. The cells which make up the 

 inner row of this layer may be described as fusiform, oval, or 

 rhomboidal in shape; they generally show a distinct oval nu- 

 cleus, in which also a distinct spot-like nucleolus is visible; they 

 vary in length from o.oio millim. to 0.015 miHini., and in width 

 from 0.002 millim. to 0.005 millim ; the connective tissue of the 

 seventh layer is attached to the inner end of some of the cells, 

 while their outer end passes off into a fibre which runs radially 

 toward the external surface of the tectum and probably extends 

 as far as to the second layer. The fibres of these cells have a 

 tendency to run in bundles formed of several united together; 

 the bulk of this stratum is made up of cells of a smaller size than 

 those described, and generally more rounded ; these are attached 

 to the fibres of the former cells, sometimes by a short stalk and 

 in other cases they are sessile ; they usually measure 0.004 millim 

 by 0.003 milliiti-, with but little variation in size. There is pres- 

 ent in most cases a distinct nucleus, which is generally rounded, 

 but sometimes it presents a tendency to the oval form. About 



