70 
nEOKOE E. NICHOLES. 
its reception on the nnder surface of that fold (c.f. Text-fig. 
6, r./.). 
It is exceedingly diflficnit to recognise the fibre in transverse 
sections of the brain; in the canalis centralis it may, 
however, be made out, but it is very fine ; indeed, its diameter 
probably nowhere exceeds 0'3 /m, which is its measurement 
at the hinder end of the fourth ventricle. 
The next stage of which I have sections is represented by 
an embryo of 30 mm. cut sagittally, and a slightly older 
specimen (40 ram.) cut transversely. 
In these, beyond the increase in size of the various parts, 
there has not been a great deal of change (figs. 42, 46). Tlie 
choroid plexus of the mid-brain has grown forward so that it 
now overlies the hinder part of the posterior commissure, and 
has a well-marked ventrally projecting median fold, much as 
Dendy has described (’02) for the ammococte of Geotria. 
(Tliis^ fold was barely indicated in tny smaller specimens.) 
'I'he posterior commissure still shows no indication of any 
downfolding, and remains extended horizontally in an antero- 
posterior direction, on a level with the top of the habenular 
ganglia. 
The sub-commissnral organ has increased considerably in 
size and its two halves are now seen to be somewhat widely 
separated (fig. 42, .<?. c. o.). At its anterior end the left half 
of the organ extends forward a short distance in advance of 
that upon the right side. Posteriorly the specialised ependy- 
mal epithelium now extends a little way behind the posterior 
commissure upon the side walls of the mid-brain in a manner 
exactly recalling the condition of the aramocoete described 
and figured by Dendy (’02, p. 490). 
Reissner’s fibre, too, shows a marked increase in size, 
having, in the 30 mm. specimen, a diameter of approximately 
0'8/i, measured in the fourth ventricle. The lesser fibres 
have joined up beneath the posterior commissure into a jmir 
of fibres which remain distinct for some distance, and seem 
to unite to constitute the single fibre just anterior to the 
rhombo-mesencephalic fold. In transverse sections the 
