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 19 (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 (coms. 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. Tuer Trcorum OPpticum. 
Since the tectum is that portion of the brain 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 from 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 (sw. tet. 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, which 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. 
— 
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