STKyCTURE AND DEVELOPMENT OF REISSNER’s FIBRE. 33 
by those of Sargent’s works which liad at that time been 
recently pnblislied. It must accordingly be borne in mind 
that in April, 1901, when the second of Sargent’s preliminary 
papers appeared, that author had not apparently attempted 
to retrieve the mistakes consequent on his initial errors of 
identification. It is clear, too, from Sargent’s descriptions 
(’01, p. 447-448) and figure (pi. 2, fig. 9) of the condition of 
Reissner’s fibre in Elasmobranchs (as studied by him in very 
young Raia erinacea), that he had not at that time dis- 
covered how very far forward the fibre really extends beneath 
the posterior commissure, which structure, indeed, with its 
related sub-commissural organ (ependymal groove), he did 
not, even then, so much as mention. 
Into practically identical errors both of omission and of 
commission, Houser, as I shall proceed to show, also fell. 
Indeed, it would appear that upon this subject Houser has 
blindly followed the fii-st lead given by Sargent, while 
Sargent himself, when giving in his last paper (’04) an account 
that differs essentially in cei'tain important details from his 
earlier accounts, yet claims (’04, p. 164) that “ Houser 
has fully confirmed my results as set forth in m\^ preliminary 
papers ” ! 
Although describing, in Reissner’s fibre and its alleged 
cellular connections in the brain, a structure hitherto unde- 
scribed in adult Selachians, Houser failed to give any 
illustration which would enable his readers to gatlier clearly 
at precisely what point “the well-marked tract” of fibies 
“emerges from the mid-brain roof to penetrate the aqueduct, 
of Sylvius as the fibre of Reissner” (op. cit., p. 130). Indeed, 
in none of his numerous figures does he represent Reissuer’s 
fibre at all, and the absence, in particulai-, of any illustration 
showing precisely where he believes Reissner’s fibre to 
emerge from the mid-brain sul)stance into the aqueduct of 
Svlvius is the more to be regretted on account of the vague- 
ness of his descriptions. 
As is well known, the posterior commissure is included by 
many authors (Edinger, ’08, Johnston, ’U7) in the mid-brain, 
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