STRUCTURE AND DEVELOPMENT OP REISSNER’s FIBRE. 115 
Fig. 34. — P. fluviatilis. Part of anotRer transverse section, taken 
at a point immediately anterior to the posterior commissure. X 30. 
Fig. 35.— P. fluviatilis. Part of another transverse section, through 
the postei’ior commissure, showing the right diaccelic recess. X 30. 
Fig. 36. — P. fluviatilis. Another ti’ans verse section through the 
mid-brain, immediately behind the posterior commissure, showing the 
end of the sub-commissural organ, x 30. 
Fig. 37. — P. fluviatilis. Horizontal section through the sub- 
commissural organ, showing the paired character of Reissner’s fibre. 
(Some of the lesser branches were added from adjacent sections.) 
X 240. 
Fig. 38. — P. fluviatilis. — Pai't of a transverse section through the 
posterior commissure, showing the sub-commissural organ and Reiss- 
ner’s fibre, cut obliquely, lying closely against it on each side. X 240. 
Fig. 39. — P. fluviatilis. Part of a horizontal section thi'ough the 
sub- commissural organ anterior to the posterior commissure, showing the 
fibrillse of Reissner’s fibre arising from the surface of the organ. X 525. 
Fig. 40. — P. fluviatilis. Median section through the extremity of 
the filum terminale, showing the canalis centralis opening 
widely by the terminal neural pore in the sinus terminal is. (The 
figure was obtained by the superposition of di’awings of several adjacent 
sections which were cut slightly obliquely to the sagittal plane.) 
X 160. 
Fig. 41.— Ichthyomyzon (Entosphenus) tridentatus. A ti-ans- 
verse section through the developing sub-commissural organ of an 
ammocoete 12 mm. long. (The level of the section is indicated by the 
line 41 in fig. 45.) X 560. 
Fig. 42. — I. tridentatus. A transverse section through the sub- 
commissural organ of an ammoccete 40 mm. long. The level of the 
section is approximately indicated by the line 42 in fig. 46.) x 320. 
Fig. 43. — I. tridentatus. A transverse section through the sub- 
commissural organ of an ammoccete 95 mm. long, x 300. 
Fig. 44. — I. tridentatus. — Part of a transverse section through 
the hinder part of the raid-brain of an ammoccete, 65 mm. long, showing 
the paired character of the isthmic canal. 
PLATE 5. 
Fig. 45. — Ichthyomyzon tridentatus. Part of a sagittal section 
through the roof of the brain of an ammoccete 14'5 mm. long, showing 
the extent of the posterior commissure and the sub-commissural organ. 
X 525. 
