STRUCTURE AND DEVELOPMENT OF EEISSNER’s FIBRE. 51 
ally the appearance of passing out towards the iter. This 
appearance is to be observed more frequently in transverse 
sections through the anterior parts of the posterior com- 
missure, where the sections cut its antero- ventral surface very 
obliquely. 
Such a condition is represented in fig-. 38, where all the 
inore conspicuous fibres have been carefully drawn in, with 
the aid of a camera lucida, from an actual section which 
formed part of a complete series stained with iron-brazilin. 
As one would naturally expect, these fibres could be followed 
from section to section, past the point where they might 
have appeai-ed to emerge, to the side of the posterior com- 
missure remote from that upon which their related cells lay. 
It was, perhaps, his failure to interpret correctly appear- 
ances such as these that led Sargent to suppose that he 
could trace axons into the ventricle; while the obliquely cut 
sections of Keissner^s fibre, to which I have already referred 
as seeming to penetrate the ependymal epithelium, may have 
been regarded as the continuations of such axons. 
The utter improbability of such an origin for the fibi*e of 
Reissner, from the confluence of a large number of ueuraxons, 
will become apparent when it is stated that in many instances 
an individual fibre from this laterally placed group of cells 
has a diameter as great as, or even greater than, that of the 
entire Reissner’s fibre itself, while collectively (and they ai-e 
quite numerous) they would vastly exceed that structure in size. 
Furthermore, according to Sargent, these constituent fibres 
form but a fraction of the entire fibre in Petromyzon, for 
other “axons” are described as entering into it (1) from a 
paired nucleus of large cells, which he correctly homologises 
with the “Dachkern,” and whicli lies dorsal or dorso-lateral 
to the posterior commissure, and (2) from large multipolar 
cells ill the habenular ganglia. 
As a matter of fact, the ultimate factors of Reissner’s fibre 
aie exceedingly fine, and can be traced only to the free 
surface of the elongated cells of the sub-commissural organ 
(fig- fh.). 
