192 BULLETIN : MUSEUM OF COMPAEATIVE ZOOLOGY, 



be homologous with the cells of the * Dachkeni ' described by Rohon 

 in selacliians. 



Johnston (:01, p. 50), in his magnificent monograph on the brain of 

 Acipenser rubicixndus, has described this group of cells, under the name 

 of 'nucleus magnocellularis,' ■'^ as located "in the cephalic part of the 

 tectum, at either side of the mid-dorsal line." "These cells are never 

 impregnated in Golgi preparations, so that it is impossible to study their 

 dendrites satisfactorily, or to determine whether they have neurites. 

 A few of the cells are found as far caudally as the middle of the tectum, 

 but the great majority are situated dorsal to the posterior commissure, 

 suiTounditig a blind pouch of the aqueduct whicli extends forward above 

 the commissure. The cell body is usually pear-shaped with a single 

 large process from the smaller end. Tiiere may be two or more smaller 

 processes from other parts of the cell. The smaller end of the cell is 

 usually turned away from the cavity and the large process arising from 

 this end of the cell turns at once laterally and can be traced for some 

 distance in hematoxylin sections as a very thick fibre running just over 

 the ependyma cells and among the nerve cells of the inner zone." 



B. Critical Discussion. 



The large process described by Johnston in Acipenser is probably the 

 axon of the cell which goes to form Reissner's fibre, and in young larvae 

 would doubtless have been found passing directly into the ventricle ; but 

 in the adult state, as in other ganoids, it takes a tortuous path before 

 emerging into the ventricle in the median plane. As Reissner's fibre 

 was not observed, and only adult brains were studied, it is not surprising 

 that Johnston failed to recognize the true significance of these cells. 



Though Johnston ( :01, pp. 72, 73) was unable to follow the processes 



1 Johnston is the first to apply the term ' nucleus niagnocellularis ' to this group 

 of cells, though Edinger (:01) almost simultaneously used the term 'nucleus niag- 

 nocellularis tecti' in connection with the homologous group of cells in Scyllium. 

 Though these terms, particularly in the latter form, describe this nucleus ade- 

 quately for the animals studied by Johnston and Edinger, it is not equally appli- 

 cable to other groups, such as the teleosts and mammals, where the cells are not of 

 unusual size. Moreover, these terms are liable to confusion with similar terms 

 long used in connection with other cell groups. Reissner described a 'nucleus 

 magnus' lying in tlie posterior part of tlie optic lobes (see Bellonci, '88, p. 26; 

 and Haller, :00, p. 298). Stieda described the 'nucleus magnus thalami.' A 'nu- 

 cleus magnocellularis diffusus ' has been described in the medulla, ventrad to the 

 posterior end of the 4th ventricle (see Rabl-Riickhard, '94, p. 711; also Kulliker, 

 '96, pp. 323, 325). 



