72 
GEOKCE E. NTCnOLLS. 
seems to be becoming downfolded as tlie plica meso- 
prosencephalica. It is, however, still a comparatively 
tliin band of fibres, the upper and lower surfaces of which, as 
seen in sagittal section, are horizontal. 
Upon the ventricular surface of the sub-commissural organ 
in some sections fairly numerous fibrillte of Reissner’s fibre 
may be distinguished. The fibre itself passes backwards 
beneath the rhombo-mesencephalic fold as a single thread, 
which traverses a deep isthmic canal upon the ventral surface 
of that fold. In this canal Reissner’s fibre lies quite freely, 
d'lie diameter of the fibre in the fourth ventricle of this 
specimen is little more than 1 /i. 
The study of the posterior end of the fibre is attended with 
much more difficulty on account of the exceedingly minute 
lumen of the canalis centralis towards its hinder end. 
The spinal cord has been stated to arise in C^'clostomes and 
in 'I’eleosts as a solid cord, its lumen appearing subseqnenth'. 
In my youngest specimens the lumen has already appeared, 
and the spinal cord has assumed the appearance characteristic 
of early vertebrate embryos. In its hinder part the spinal 
cord is almost cylindrical, and has accordingly, in transverse 
sections, a nearly circular outline. Owing to the thickness 
of the walls laterally and to the relative thinness of the median 
zone the canalis centralis appears in such sections as a 
narrow vertical cleft, which is almost obliterated near the 
middle of its height. Below it widens into a tiny circular 
space, and sometimes similarly widens slightly above (fig. 
47, C.C.). 
In the 12 mm. larva this central canal has a maximum 
transverse diameter of little more than 4 p, measured at a 
point about half a millimetre from the end of the bod)'. 
Forward of that point the lumen is occasionally blocked by 
an intrusive cell, but behind the canal is entirely free from 
any such contents. 
The canalis centralis extends to the hind end of the 
body, appearing even in the section last but two of this 
serie.s, which was cut transversely in sections 10 p in thickness. 
