STECCTURE AND DEVELOPMENT OP REISSNEli’s FIBRE. 103 
wish, aud do not represent at all adequately this wonderful 
mass of fibie; it was, however, a particularly difficult subject 
to photograph, both on account of the staining and also on 
account of the thickness of the sections (20 |i). 
Some idea of the relatively enormous size of this mass of 
coiled fibre may be formed when it is stated that it appears 
in four or five adjacent sagittal sections, each 20 ju in thick- 
ness. It thus probably had a thickness of not less than 80 n 
from side to side. Measured dorsi-ventrally the mass has a 
length of close upon 100 fi, while its average antero-posterior 
diameter is nearly 80 p. 
'J'he canalis centralis a little forward of the sinus 
terminal is is empty of fibre, and I estimate that the 
inulbeny-like mass represents some 200 to 250 mm. of 
fibre retracted into a heap barely O'l mm. high. Since the 
specitnen was little over a foot in length I can only suppose 
that the fibre must have snapped at a point very far forward. 
This breakage must have been followed by a continuous 
recod of the hinder piece into the s i n us terminalis, such 
recoil having been completed before fixation was effected. 
Sagittal sections through the tail of a second specimen 
appeared to show a somewhat similar condition, but the 
sections were so crumpled and distorted in this region that 
it is not possible to speak with certainty. 
Such a condition is, however, undoubtedly of frequent 
occurrence (Sanders ffi4, Studnicka, ’99), and must be the 
result of a breakage of the fibre at some time shortly before 
fixation. It is certainly not the condition of the functional 
fibre, which should be iuseried, as in the Petromyzontidae, 
as a tautly stretched thread, into the meningeal portion of 
the wall of the sinus terminalis. This normal condition 
has, so far as I can discover, been observed only by Sanders 
(’94, p. 11). 
Critical Discussion. 
As already noted, an account of the occurrence and rela- 
tions of Keissner’s fibre in Myxine was given by Sanders as 
