STRUUTUlili] AND DEVELOPMENT OF REISSNEli’s FIBRE. 79 
ammocccte of Petroiuyzon planeri wifcli those of Sterzi 
(’07^ figs. 99 and 124-6) for tlie same ammocccte^ and also of 
that of P. flu via til is; or with iny figures (figs. 45, 46, and 
Text-fig. 6) of the corresponding structures in Ichthyo- 
niy zon (Entosphenus) tridentatus will, I think, convince 
the reader that what Sargent has named the posterior com- 
missure (’04, pi. i, fig. 2) is undoubtedly the superior 
(habenular) commissure, while that part of the braiu which 
he labels tectum opticum clearly represents the region 
of the posterior commissure. 
Indeed, a comparison of Sargent’s own figures (op. cit., 
])1. i, figs. 2 and 4) bears out this conclusion. In his fig. 4 
the epiphysis is seen, cut in transverse section, dorsal to the 
habenular ganglion, which is perfectly correct for very young 
larvae (cf. Sterzi, op. cit., fig. 122). In these young speci- 
mens it may also extend backwards to a slight extent, dorsal 
to the posterior commissure. It continues, however, to grow 
forward, and, as I have pointed out above, has already, even 
in my youngest s})ecimens, attained its full forward displace- « 
ment relative to the other parts of the brain, lying above the 
anterior region of the dorsal sac. 
In his fig. 2, however, Sargent represents it extending 
dorsally far backwards, behind the posterior commissure, and 
altogether posterior to the habenular ganglion, a quite 
unnatural position if the parts were correctly identified. 
Further, the posterior commissure reaches a size ^ and shape 
comparable to that of the body so labelled in Sargent’s 
figures only comparatively late in development, by which 
time the epiphysis has acquired its well-developed eye-like 
appearance aud has an elongated stalk. The right habenular 
ganglion, on the other hand, has a veiy precocious develop- 
ment and early reaches a large size, presenting at this age 
precisely the appearance and relations of the structure which 
Sargent has named the posterior commissure. 
' Even ill larva) of Ichthyoinyzou (Entosplienns) triilcntatus 
1)5 nini. long, the posterior commissure had not such a relatively con- 
siderable thickness as compared with its length. 
