WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 39 
mus bundle. In parasagittal sections the cut ends of this portion of 
the tract appear to be pointing into the thalamus. But no one of these 
authors has described fibrillations or cell endings for this thalamus 
bundle, and the absence of degeneration in Krause’s experiment would 
indicate that Mayser’s thalamus root was non-optic. 
A frontal section (Plate 4, Fig. 20) shows the relation of the thalamus 
ganglia to the tectum. The geniculate bodies lie anterior to the lobes 
of the tectum, and between them are the ganglia habenule (gn. habd.), 
which bound the third ventricle, and are separated from each other by 
the pineal-gland region. A few sections dorsal to the one shown in this 
figure the habenular commissure appears. 
As Haller (98) has found in the case of Salmo, the habenule are 
symmetrical, in the young fish at least. Because of the want of sym- 
metry in older brains it is impossible to obtain single sections in which 
one is certain that the habenule are cut in like planes. In a cross sec- 
tion which passes through both ganglia the left ganglion has a greater 
dorso-ventral diameter than has the right, while the right ganglion 
measures more from side to side than the left. 
In Figure 20 the fibres of the two parts of the optic tracts are shown 
in cross-section behind the edges of the geniculate bodies. Also behind 
the geniculate bodies lie large cells which belong to the nidulus corti- 
calis of Fritsch, the “‘ Dachkern ” of Edinger and others. 
Since fibres from this nidulus enter the tectum, I will describe its loca- 
tion more particularly in the two Pleuronectide studied. There are two 
symmetrically placed groups of very large ganglionic cells lying at the 
front part of the tectum ; they extend anteriorly from the angle of the 
optic ventricles, where the lobe of the tectum and the axial portion of 
the midbrain meet, to the outer surface of the brain above and outside 
the geniculate bodies. There is no difficulty in identifying the cells 
of the nidulus (nid. ctx., Plate 5, Fig. 23), as they are pear-shaped and 
many times larger than those of the gray layer of the tectum, into which 
the posterior portion of the nidulus extends. 
The nucleus lies in the blunt end of the pear-shaped cell, at the end 
’ opposite the coarse cell process. Since these processes gather into 
bundles in the middle layers of the tectum, the nucleated ends of the 
cells are directed towards the surface when the cells are more super- 
ficial, but toward the optic ventricles if they are deep (compare 
Fig. 22). 
There is a similar nidulus, consisting of a few (20-30) even larger 
cells, which lies ventral and exterior to the nidulus corticalis; it lies 
