WILLIAMS: MIGKATION 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 habenulse (jjn. hah.), 

 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 habenulse 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 habenulse 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 moi-e from side to side than the left. 



In Figure 20 the fibres of the two pai'ts 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 Pleuronectidaj 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 difl&culty in identifying the cells 

 of the nidulus {nid. ctx., Plate 5, Fig. 23), as they are pear-shaped aud 

 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 



