STRUCTURE AND DEVELOPMENT OP EEISSNER’s FIBRE, 83 
sagittal plane ai-e not easily prepared, and a very si ight 
obliquity might cause the open isthmic canal to Hppear as an 
exceedingly fine tube scarcely larger than the fibre traversing 
it. Even if the sections were, however, cut perfectly truly, 
the whole isthmic canal might easily lie within the thickness 
of a single section, for the full width of the double canal at the 
point shown in my fig. 10 is less than 6 /a, while behind that 
point it narrows considerably. 
Thus, to determine with certainty whether or no the fibre 
lies freely, it is necessaiy to examine sections cut transversely 
in this region. Even so, it does not follow that the sections 
so cut will show the fibre, for unless they are prepared with 
especial care it is more than likely that pieces of fibre may 
become displaced or lost. It is, therefore, of interest that 
in the one series which Studnicka examined which was cut 
transversely he found the fibre free (^99 p. 7). This par- 
ticular series of sections happened to be one through the brain 
of an ammocoete, and Studnicka concluded that the inclusion 
of the fibre within the substance of the })lica rhombo- 
m e sen ce f)h al ica must take place later in life as a conse- 
quence of the downgrowth of the bi-ain in this region. 
VVdiile, of course, there is nothing inherently iin])robable in 
the supposition that the ependymal epithelium of the edges 
of the isthmic canal should fuse ventrally, beneath the fibre, 
to fonn a tube open at either ejid,^ I see no re§.son to suppose 
that such a condition actually arises in the Peti-omyzontid£e, 
and I regard Sargent’s statements merely as <i result of 
his misiuterpretatiou of the condition observed in sagittal 
sections. 
1 have desciibed the occurrence of large snarls in the 
I’ecessns iiif rapinealis and beneath the posterior com- 
missure (Geotria), showing that the fibre, when cut 
])Osteriorly, is free to spring forward from its broken end in 
the centi’al canal of the spinal cord, or equally, breaking free 
from its attachment to the sub-commissural organ, to spring 
' Such an inclusion of the fibi’e within a more or less wide isthmic 
canal tube actually occurs in the Myxinoids (see below). 
