STEUCTCJRE AND DEVELOPMENT OF REISSNER’s FIBRE. 35 
were supposed to emerge. These latter, therefore (which had 
constituted the whole of the fibre in Raia as originally 
described by Sargent, and in Mnstelus according to Houser), 
could form but a fraction of the entire fibre, so that the 
difficulty of accommodating these several fibre-tracts within 
the compass of a single thread whose diameter is little more 
than that of a coarse nerve-fibre is increased rather than 
diminished. 
The condition of the adult Mustelus described by Sargent 
('04) thus differs essentially from the condition that this 
author had previously (’01) figured and described for Raia, 
yet Houser, in the meanwhile (’01), had claimed to have found 
the fibre in the adult Mustelus in preciselj' that condition 
which Sargent had described for Raia. 
Again, Sargent records that “ the exact method by which 
the fascicles enter the ventricle and form Reissner’s fibre has 
been difficult to make out in Raia, the connections having 
been broken away in all my series of sections” (’04, p. 169). 
He further admits (’04, p. 173) that in Mustelus, too, “ near 
the upper limit of the ependymal groove the fibres are lost, 
but apparently they pass between the ependymal cells into the 
groove,” and “a direct connection between the fibre-tracts 
described and Reissner’s fibre has not been observed in this 
species.” Thus it would appear that Sargent never actually 
saw in Elasmobranchs that emergence of the constituent axons 
“between the ependymal cells” whicli both he and Houser so 
confidently describe. That Houser, likewise, did not see this 
“emergence ” of these “ fibi'e-tracts ” is probable, for in this 
connection it is not without significance that Houser alto- 
gether omits (just as Sargent before him had done) all 
mention of the posterior commissure, and entirely overlooks 
the sub-commissural organ (ependymal groove), to which 
Sargent had not at that time directed attention, although the 
latter author afterwards described this structure and con- 
sidered it to be developed as an anchorage and support for 
Reissner’s fibre. This oversight on the part of Houser is the 
more astounding seeing that he devotes a considerable part 
