194 bulletin: museum of comparative zoology. 



which were impregnated in his preparations, lying near the median 

 plane and bordering on the ventricle. He designates these as, — " Type 

 C : cells of the torus longitudinalis Halleri," and believes them homol- 

 ogous with the cells of the torus described by Sala.^ This conclusion 

 must be set aside, as the disposition of the axons and dendrites oC these 

 two classes of cells is not identical; and moreover I believe I have 

 conclusively shown that it is the cells of the * nucleus magnocellularis ' 

 of Johnston, lying in the same region, which are homologous with Sala's 

 torus cells. 



C. Summary for Ganoids. 



The ganoid brain is relatively simple in structure and presents a 

 rather generalized type. It is, therefore, an excellent starting-point 

 for almost any investigation in comparative vertebrate neurology, and 

 has been found especially so in the study of the endings of the optic 

 reflex apparatus. 



The apparatus is late in development in ganoids, and is not fully 

 established until several days after hatching. The cells, 40 to 100 in 

 number, lie close to the ventricle in the anterior portion of the tectum 

 opticum in, or near, the median plane, and for the most part are grouped 

 about the margin of that portion of the ventricle which extends above 

 the posterior commissure. At an early stage of larval development 

 these cells send tlieir axons separately and directly into the ventricle of 

 the optic lobes. The axons, growing caudad through the fluid-filled ven- 

 tricle, come together and coalesce, forming lieissner's fibre. In later de- 

 velopment this coalescence progresses anteriorly, even into the tectum, so 

 that the number of divisions of Reissner's fibre in the ventricle is greatly 

 reduced. The fibres of the posterior commissure, developing after the 

 axons of the optic reflex cells have emerged into the ventricle, usually 

 take a course anterior and ventral to the cells, but in Polypterus some 

 of the cells are ventral to the commissure. 



In adult ganoids, the development of the dorsal decussation of the 

 tectum across the thin median roof of the mesencephalon has crowded 

 downward the large cells of the optic reflex apparatus ('nucleus magno- 

 cellularis' of Johnston) so that they form two elongated prominences 

 separated by the median fissure. These ridges, the homologues of the 

 lobes of the torus longitudinalis of the teleost brain, do not appear in 

 the larval ganoid brain, and in adult Amia are scarcely distinguishable. 



1 There is no authority for the application ' Halleri,' as the term torus longitu- 

 dinalis was first used by Stieda ('68, pp. 24 et seq.). 



