68 BULLETIN OF THE LABORATORIES 



"Then follows a layer of gray matter, with associated fibres. Be- 

 tween the prominences is a considerable commissure of fibres which 

 lose themselves among the longitudinal fibres." 



" The fibres on the surface of the corpora quadrigemina I unhesi- 

 tatingly regard as the real roots of the optic nerve."' 



(In this latter statement Steida is doubtless mistaken.) 



The sections figured on Plate VIII, Figs, i, 2, and 3, and Plate 

 IX, Figs. 1-5, will convey an idea of the changes observed in success- 

 ive sections. 



Fig. I, Plate VIII, indicates the structure of sections at the pos- 

 terior commissure. The nates above are cut near the anterior (cephal- 

 ad) border. They are at this point receiving fibres from the latero- 

 dorsal optic tract. Some fibres associated with the latter, however, 

 dijj downward and medianly to interdigitating masses of gray matter 

 in the lateral aspects of the posterior commissure, Fig. 2, c. (See Fig. 

 7.) Above the commissure is a protuberance of gray matter, looking 

 toward the inter-natal cleft, which resembles the nates and seems to 

 have no special significance. Below the commissure are masses of 

 gray matter, composed of small cells of various forms, though chiefly 

 pyramidal, with long processes, which in position as nearly resemble 

 the coUiculi as any structures of lower vertebrates. The bundles en- 

 tering the posterior commissure diverge rapidly and arch over the last 

 mentioned body, soon passing caudad out of the section. The poste- 

 rior commissure seems not to be a true commissure but, in part at 

 least, a decussation. 



In the median line below the aqueduct there is a collection of 

 cells (Fig. 9,) of the fusiform variety, occupying the position of the 

 raphe. Beneath the optic tract is a large mass of gray substance (cor. 

 pus geniculatum externum) perforated at intervals by well-defined 

 tracts, which externally lead obliquely toward the periphery and medi. 

 anly dorsad. The cells are scattered and small, but very acutely an- 

 gular and of a multipolar variety. A similar body (corpus genicu- 

 latum internum) lies below it and only separated by a tract of irregular 

 fibres. Its fibres pass cephalad and medianly. Still another small 

 but rather definite nucleus lies in the latero-ventral region within the 

 longitudinal ventral bundle, which here is peripheral. The entire 

 central portion {formatio reticularis)^ is a reticulum of cells and fibres 

 surrounding longitudinal disperse fascicles. A few very large multi- 

 polar cells are scattered in the meshes with the peculiar radiating con- 



