40 bulletin: museum of comparative zoology. 



posterior to, but in contact with the optic tract. This possibly is the 

 nidulus anterior of Edinger, though I have traced no fibres from it. A 

 few cells of this nidulus are shown between the two portions of the tract 

 in Figure 1 9 (Plate 4) . 



In one instance I found a cell of the nidulus corticalis which sent a 

 fine process, probably a neurite, ventrad with the other fibres of the 

 optic tract (Plate 5, Fig. 22), This could be followed nearly to the 

 chiasma, but whether it continued to the eye or bent backwards into 

 one of the post-optic commissures, I cannot say. 



I can confirm C. L. Herrick ('91-'92) in his statement that the com- 

 missura horizontalis (corns, hz., Plate 5, Fig. 22) arises from the nidulus 

 corticalis. The fibres forming this bundle were fine and took the same 

 quality of Golgi impregnation as the single fibre just described from 

 one of the cells of the same nidulus which passed downward through the 

 tractus opticus. The fibres composing this bundle can be followed in 

 two or three parasagittal sections to the nucleus rotundum of the same 

 side ; they pass through this nucleus, and then turn forward and cross 

 to the opposite side behind the chiasma as the horizontal commissure. 



4. The Tectum Opticum. 



Since the tectum is that portion of the bi-ain in which the optic 

 tracts terminate, it should be the place in which the transition from 

 sensory to association or motor neurons takes place. 



There are certain points of interest which can be shown fi'om a sur- 

 face view. At the anterior ends of the tectal lobes, in P. americanus, 

 but not in Bothus, there is an exterior furrow or sulcus (sul. tct. opt., 

 Plate 2, Fig. 11), much like one that is found in the cerebrum of simple 

 type — in that of a turtle, for example. This gradually disappears toward 

 the posterior region of the tectum. Cross-sections in the anterior region 

 show that this sulcus is due to a lateral horizontal depression in each 

 optic lobe, which divides it into almost equal dorsal and ventral parts. 

 The ventral portion of the tractus supplies the ventral half of the lobe 

 and the dorsal portion the dorsal half. The geniculate bodies lie in the 

 region of greatest constriction of the tectum. 



For convenience, I divide the tectum into seven layers, indicated by 

 the numerals 1-7 (Plate 5, Figs. 22, 23), in addition to the membranes 

 of the brain, wliich are the vascular connective-tissue layer (the arach- 

 noid, mb. ach.) and, beneath this, a very thin membrane, the pia, to 

 which the endings of the ependymal cells reach, and along which is 

 found here and there a nucleus. 



