36 
GEORGE E. NIOHOLLS. 
of his p:iper to an account of the supporting elements 
(ependyma and neuroglia) in the various regions of the brain, 
and that it happens that the snb-commissural organ reaches 
nearly, if not quite, its maximum development in Selachians 
(see Text-figs. 2, a,b, and 3, A, and figs. 2, 3). That any observer, 
especially one professing to make a study of supporting 
elements, could have seen the emergence of the constituent 
axons ” or “fibre-tracts” (so-called) of Reissner’s fibre into 
the mesocoel, and yet have failed to notice this extraordinarily 
developed ependymal epithelium is almost incredible.^ 
Concerning the “ Dachkern ” Honser remarks: “It has 
remained for Saigent (1900) to show that not only is the 
roof nucleus present in all vertebrates but that it is a part 
of a most interesting mechanism, the fibre of Reissner” 
(’01, p. 132). This statement is by no means correct, foi- 
although the existence of the cells of this “roof nucleus” had 
been described in some cases long’ before in several classes 
of vertebi-ates, yet, so far as I can find, this nucleus has 
never been identified in mammals, nor at that time had it 
been identified in Teleosts. Subsequently Sargent stated 
that the torus 1 on gi t n d i n al i s was the homologue of 
' Studnic'ka (’00), in the previous year, had described and figured this 
modified ependymal epitlielium in Scyllium, I’emarking that he was not 
aide to determine its function, while, so far back as 1892, Edinger (’92) 
had called attention to it, suggesting that it might have some secretory 
function. 
Text-fig. 2. — Slightly diagrammatic median sagittal sections 
through the mid-brains of (A) Raia blanda; (B) Scyllium 
eanicula; (c) Polypterus sp. ; (d) Amia calva ; (e) Esox 
lucius; to show the appai’ent shifting of the position of the 
“ Dachkeni " in fishes. [(D) Has been borrowed from Sai’gent's 
("04) paper, while in (c) the outline has been taken from a 
figure by Graham Kerr into which the “Dachkern” cells have 
been drawn from the figure by Sargent ('04, PI. VI, fig. 41).] 
Dk. “ Dachkern.” Ii.f/. Habenular ganglion, iiif. Infra-pineal 
recess, iii.r. Mesocoelic recess, p.c. Posterior commissure. 
r.f. Reissner's fil)re. s.c.o. Sub-commissural organ, tect. mes. 
Tectum mesencephali. tor. long. Torus longitudinalis. 
vl. chm. Valvula cerelielli. (The lines aa' and cc' indicate 
approximately tlie levels at which the sections rejn’esented in 
Text- Mg. 3, A, c, were taken.) 
