114 
GEORGE E. NICHOLES. 
Fig. 20. — G. australis. Part of a median sagittal section through 
the hind-brain (from one of Prof. Dendy’s preparations). A great 
length of Reissner’s fibi’e is seen lying in the fourth ventricle and 
canalis centralis (somewhat displaced). X 150. 
PLATE 3. 
Fig. 21. — Bdellostoma (Polistotrema) stouti. Paid of a 
median sagittal section through the mid-brain. X 50. 
Fig. 22. — B. stouti. Part of a nearly median sagittal section 
through the mid-brain of another specimen, showing the optocoel much 
more reduced. Reissner’s fibi’e is seen issuing from the sub-commissural 
canal. X 60. 
Figs. 23-25. — B. stouti. Portions of transverse sections through 
the mid-brain of another specimen. These are taken approximately 
at the levels indicated by the lines 23, 24, 25 in Text-fig. 7. X 68. 
Figs. 26, 27. — Myxine glutinosa. Portions of transverse sections 
through the mid-brain, coiresponding roughly to those shown for 
Bdellostoma in figs. 23, 25. The cavities in Myxine are, however, 
relatively much smaller. Fig. 26 x 115. Fig. 27 x 200. 
Fig. 28. — M. glutinosa. Part of a transverse section through the 
spiual cord, showing the double character of the canalis centralis 
and Reissner’s fibre in the lower of the two canals. X 350. 
Fig. 29. — M. glutinosa. Part of a sagittal section through the 
spinal cord. X 60. 
Fig. 30. — M. glutinosa. — Part of a sagittal section through the 
hind-brain and spinal cord, showing the large space (x) situated in 
the hind-brain above the canal-like portion of the fourth ventricle. 
X 150. 
Fig. 31. — Bdellostoma stouti. Part of a transverse section 
through the spinal cord for comparison with that of Myxine (fig. 28). 
X 450. 
Fig. 32. — B. stouti. Pai’t of a sagittal section through the spinal 
cord showing Reissner's fibre in the lower canal, x 68. 
PLATE 4. 
Fig. 33. — Petromyzon fluviatilis. Part of a transverse section 
thro\igh the thalamencephalon, showing the sub-commissural organ (on 
the true left side) between the right habenular ganglion and the left 
optic thalamus. X 30. 
