BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI. 151 
floor resembles very closely a section of columnar epithelium. 
The roof of the cavity is from two to three times as thick as 
the floor, and is composed of two layers, (1) an inner lining 
layer, similar to the floor, except that the cells are not placed 
so close together, and, in consequence, some lie over on their 
sides, and the columnar effect is not so marked; and (2) an 
outer (dorsal) layer, containing a great many ependyma cells 
lying separate from each other, with their long axes in all 
planes. This dorsal layer also contains occasional large 
round cells, with round nuclei. Near the caudal end of the 
roof there is a small evagination, directed forward (fig. 13) ; 
the dorsal layer just referred to appears to be a prolongation 
of this evagination. In places there is a space between these 
two layers, in other places they are held firmly together by 
transfixing processes of ependyma cells. Immediately caudal 
to the stalk the two layers are fused together and merged 
completely into one. The canal of the stalk, instead of being 
placed in the centre, lies just behind the anterior surface, so 
that the cephalic wall of the stalk is the continuation of the 
thin floor of the process, and the caudal wall the continuation 
of the thick roof. The infundibular process les embedded 
among the thick fibres of the membranous cranium, and 
directly beneath it, though separated from it by a layer of 
fibrous tissue, is the hypophysis. This consists of a much 
coiled tube, probably glandular (Retzius, 1895), but I have 
not been able to find any outlet for it, nor to determine the 
cell structure. The cells themselves have not stained at all, 
or only with the faintest tinge, in my preparations, so that I 
cannot tell their shape or size. The nuclei, however, which 
are fairly large, have stained intensely black or deep blue, 
so that the hypophysis in section looks like a collection of 
more or less rounded bodies filled with black granules (fig. 
13). 
A saccus vasculosus does not exist in Bdellostoma, but the 
elements out of which it develops in higher forms are there, 
and Bdellostoma consequently marks an interesting stage in 
its evolution. The foundation of the nervous element lies in 
vol, 49, parr 1,—NEW SERIES. 12 
