116 
GEORGE E. NICHOLES. 
Fig. 46. — I. tridentatus. Pai-t of a sagittal section through the 
roof of the brain of an ammocoete 30 mm. long. X 320. 
Fig. 47. — I. tridentatus. Transverse section through the tail of 
an ammocoete 12 mm. long, showing the condition of the canalis 
centralis. X 420. 
Fig. 48. — I. tridentatus. A similar section through the tail of the 
same ammocoete, taken at a point 40 micra behind that represented in 
fig. 47. X 420. 
Fig. 49. — I. tridentatus. Another section from the same series, 
taken at a point 50 micra posterior to that shown in fig. 48, and distant 
only 20 micra from the actual end of the tail. It shows the canalis 
centralis widened out into a sinus terminalis. X 420. 
Fig. 50.— I. tridentatus. A median sagittal section through the 
tail of an ammocoete 14'5 mm. long, showing Reissner’s fibre ending in 
a terminal plug in the sinus terminalis. X 518. 
Fig. 51. — I. tridentatus. Median sagittal section through the tail 
of an ammocoete 42 mm. long, showing Reissner’s fibre expanded into a 
terminal plug inserted into the wall of the sinus terminalis. X 394. 
Fig. 52. — I. tridentatus. A similar sagittal section through the 
end of the tail of an ammocoete 75 mm. long, showing Reissner’s fibre 
coiled in the sinus terminalis. x 394. 
Fig. 53. — I. tridentatus. A section cut somewhat obliquely to the 
sagittal plane, showingJReissner’s fibre ending in a terminal plug in the 
sinus terminalis of an ammocoete 36 mm. long. X 690. 
Fig. 54. — I. tridentatus. A sagittal section through the tail of 
an ammocoete 105 mm. long. (Reissner’s fibre has been completed 
from one or two adjoining sections.) X 94. 
Fig. 55. — I. tridentatus. A nearly transverse section through the 
anterior part of the sinus terminalis of an ammocoete 65 mm. long. 
Reissner’s fibre hasjretracted into a coil, and some fibrous tissue appears 
to have been pulled into the sinus terminalis. X 560. 
Fig. 56. — I. tridentatus. A sagittal section through part of the 
spinal cord of an ammocoete 34 mm. long. X 560. 
Fig. 57. — Geotria australis. Part of a sagittal section through 
the mid-brain of a velasia stage showing the pair of principal factors of 
Reissner’s fibre uniting about halfway between the sub-commissural 
organ and the isthmic canal. X 105. 
Fig. 58. — G. australis. A sagittal section through the posterior 
commissure of another velasia, showing Reissner’s fibre broken and 
retracted slightly forwards. (From one of Prof. Dendy’s preparations.) 
X 104. 
