44 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY, 



followed backward for some little distance along distinct paths in layers 

 3 and 4, and the general appearance of the fibrillations farther back 

 indicates that these processes, branching continually, pass backward 

 through the tectum much farther than continuity can be directly traced. 

 A dendrite may branch and follow the fibrillar paths in eacii of the two 

 layers. 



A large system of fibres also enters the same general region of the 

 tectum from the axial part of the mid-brain ; some of these cross from 

 the opposite side of the brain in the lower part of the posterior commis- 

 sure. These fibres may constitute the most anterior portion of the cora- 

 missura mesencephali (Herrick's sylvian commissure) or, as I think 

 more likely, they may come from the motor regions, possibly Ilaller's 

 anterior connective. I have not succeeded in tracing these fibres to 

 any cells. 



In layer 4 appear the cells which are most characteristic of the tectum 

 (Fig. 22, i). They were impregnated in most of the Golgi preparations. 

 They are spindle-shaped, being much elongated in a radial direction, 

 and have fibrillations which extend outward as far as layer 2. Some- 

 times there is an impregnated process which goes from the deeper end 

 of the cell into layer 5, and sometimes there is not. Neumayer and 

 Mirto each state tiiat the neurites of these spindle cells are traceable to 

 the fillet layer and the fibrillations to the optic layer. ^lirto describes 

 cells with the same processes but witli much more slender bodies. The 

 spindle-shaped bodies are shown by my hematoxylin preparations to 

 be very abundant indeed in this layer, only a few taking tlie Golgi 

 impregnation in a single specimen. In this layer (4) there were also 

 found sparingly cells (Fig. 22, k) with rounded bodies and processes 

 which fibrillate inwards and extend into the fillet layer (5). A very few 

 pyriform cells lie near the deep surface of this layer (4) and send their 

 processes outward (Fig. 22, A). Fusari shows irregular, large-bodied 

 cells with many processes and neurites, when such are present, extending 

 into layer 5. A bifurcate cell is figured by Mirto with its telodendrites 

 in layer 3. My flounder impregnations produced neither of these types. 



I have spoken of layer 5 as the fillet layer because it is composed 

 chiefly of fibres which pass backward and medianward, forming the so- 

 called corona radiata of Gottsche, the lemniscus or fillet system. 



This layer is composed of cross and longitudinal fibres which, seen in 

 tangential section, form a meshwork over the whole of the dorsal part 

 of the tectum. In front of the optic ventricles bundles of fibres 

 (Plate 5, Fig. 22, Imn.) can be followed from the axial part of the mid 



