70 
OKOUGE E, XLCflOLTiS. 
Figure 55 represents tlie fibre of Ifeissner emerging from this 
tangle, and sliows also some of the fibrous tissue of the 
sheath drawn into tlie terminal sinus. lu all of these ca.ses 
the fibre is enormously swollen. 
On the other hand, in the specimens of 75 mm. and 105 
mm. the recoils were evidently much less extensive, the break 
in the fibre doubtless having occurred very far posteriori}". 
In the latter specimen a quantity of lymphoid matter has 
intruded through the terminal neural pore, and with this 
Reissner’s fibre (which is not greatly coiled) seems to have 
become entangled. It may possibly be that we have here a 
case of incipient reg'eneration. 
In the brain of the ammocoete of Geotria the sub-commis- 
sural organ is well developed. Its condition has already 
been described and figured by Bendy (’02, figs. 1, 2), and it 
differs from that of the ammocoete of Ichthyomyzon 
(En tosphen us) tridentatus of coi-responding age in little 
but that its two halves are more nearly apposed in the middle 
line, and that in consequence of this median position the 
lamina of the grooves are presented almost directly ventrally. 
The cilia which clothe the ventricular surface of this organ 
are short and close set, and amongst them are some longer 
undoubted fibrillm of Reissner’s fibre. I cannot, however, 
certainly identify the fibre itself in this region, but beneath 
the rhondjo-mesencephalic fold there is a shallow isthmic canal 
in which Reissner’s fibre may be made out. In the canalis 
centralis of the spinal cord it can be readily traced. Of 
the tail of this species I have had no material, so that nothing 
can be stated as to the condition of the fibre and its relation 
to the sinus t e r m i n a 1 i s. 
Critical Discussion. 
(a) T he S u b - c o m m i s s u r a 1 Organ in the Adult. 
It has already been pointed out above that in Retro my zon 
mar in us, Petromyzon fluviatilis, Ichthyomyzon 
(En tosphen us) tridentatus, Geotria australis and 
