38 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



external, ventral portion (trt. ojot. v.) passes outward and around to its 

 distribution on the posterior, lateral, and ventral tectal surfaces. 



The geniculate body (Figs. 20, 21, cp. gyiic.) lies in the angle be- 

 tween the two portions of the Y-shaped tract, but almost entirely in 

 front of their plane. There is some indication of a division of the corpus 

 geniculatum into anterior and posterior parts. 



In both Weigert and Congo-red preparations it could be seen that a few 

 optic fibres entered the geniculate bodies (Plate 4, Fig. 21). C. L. Her- 

 rick ('92, p. 430) found no ending of optic fibres before reaching the 

 tectum. This ending has been demonstrated, however, by Mayser ('81) 

 in Cyprinoids, by Auerbach ('88) in the trout, by Haller ('98) in Salmo, 

 and by Krause ('98), who used Marchi's method for degenerate nerves, 

 in Cyprinus auratus. Edinger ('96, p. 126), makes the following state- 

 ment for vertebrates. " Im Geniculatum [laterale] endet ein Theil 

 des Sehnerven mit mJichtiger Anfsplitterung, und mitten in diese Faser- 

 ung tauchen die Dendriten langgcstreckter Doppelpyramiden. Das 

 mediale Ende dieser Pyramidenzellen splittert auf in eiuem Zuge, der 

 wahrscheinlich auch dem optischen System angehort." 



I have no Golgi preparations which show optic fibres actually fibril- 

 lating in these bodies. There was, however, in the geniculate bodies 

 but one type of cell impregnated with the chrome-silver. This was a 

 small unipolar cell (Plate 5, Fig. 22) with a short process ending in 

 very thick short fibrillations directed towards the end of the geniculate 

 body into which the optic fibres enter. In a single exceptional instance, 

 a cell, otherwise like the ones described, had another short but un- 

 branched process extending in the opposite direction (see diagram of 

 tectum, Plate 5, Fig. 22, cp. gnic). 



Fusari ('87), after a study of Carassius, Macropodus, Anguilla, and 

 Lopodogaster, stated that in his opinion fibres from the tractus pass 

 through the corpus geniculatum and unite again with tlie tract to fibril- 

 late in the tectum. No preparations of P. americanus indicated such a 

 possibility. 



No other bundle of fibres could be found to leave the tract before it 

 reached the tectum itself. Mayser ('81) describes a small bundle pass- 

 ing into the thalamus at about the point of origin of the paraphysis. 

 Auerbach ('88), Mirto ('96), and Haller ('98) also indicate a thalamus 

 bundle, and Haller describes a small bundle running to the fore-brain. 

 In my opinion Mayser, Auerbach, Mirto, and Haller have mistaken a 

 portion of the ventral division of the tract, which bends outward sharply 

 in its course to the ventral posterior part of the optic lobes, for a thala- 



