22 
GEORGE E. NICHOLES. 
modified in the different classes of vertebrateSj being depen- 
dent upon the size and relations of the structures of the mid- 
brain immediately adjacent. Thus, among the forms that I 
have examined, the sub-commissural organ appears, in trans- 
verse section, as a decidedly horse-shoe-shaped structure in 
Selachians, reptiles and many mammals (PI. I, figs. 2, 3, G, 8, 
.S’, c. o.), in birds it is strongly arched (PI. I, fig. 7), and in 
amphibians (PI. I, fig. 5) perhaps somewhat less markedly 
curved, while in many Teleosts it persists merely as a flattened 
median plate which shows scarcely any trace of its originid 
jiaired character (PI. I, fig. 4). 
'Pile shape of the sub-commissural organ may be still further 
modified by transverse folding. In some cases, too, this tract 
of specialised epithelium extends around the posterior border 
of the posterior commissure on to the dorsal (posterior) 
surface of that structure, and may even line a small median 
anterior extension of the optocoel (the mesocoelic recess) 
directly above (behind) the posterior commis.snre (Text-figs. 
2, li, and 3, A, ?a.7-.). 
In man, as was pointed out by Dendy and Nicholls (’10), 
the mesocoelic recess is the sole vestige of this apparatus. It 
is connected, in foetal life, with a typical sub-commissural 
organ, which lies perfectly normally beneiith the posterior 
commissure and bears distinct evidence of its paired origin, 
but it appears sub.sequently to lose its connection with the 
general cavity of the iter, and that partof the sub-commissural 
organ which lies beneath the posterior commi.ssure altogether 
disappears. 
Between the cilia of the snb-commissural organ are found 
slender fibrilla? which are distinguishable from the ordinary 
cilia sitnply by their greater length. These collect together 
to form delicate strands which unite to constitute Reissner’s 
fibre. In longitudinal section of the typical brain (compare 
'I'ext-fig. b) the fibre is thus to be made out arising well 
forward from near the anterior border of the sub-commissural 
organ in the infra-pineal recess. It there lies closely against 
the surface of the sub-commissural organ near the middle line. 
