462 BERTRAM H. WATERS. 
being essentially primitive and in a state of degeneration, 
have gradually been more and more crowded out by the 
specialising brain development, and hence appear at a much 
later period in the ontogeny than would be expected. How- 
ever that may be, their tardy appearance is a strong proof of 
their intimate relation to the brain itself, and affords a striking 
refutation of one hypothesis which has been suggested, viz. 
that they are the mechanical results of mesoblastic pressure: 
Moreover, as Miss Platt has pointed out in her recent paper, 
the neuromeres often appear before the corresponding proto- 
vertebre, and consequeatly must be independent of any 
formative influence of the latter. 
In sections of about ten days’ incubation the eyes are most 
prominent, appearing abnormally large and apparently in 
advance of the other parts in development (see fig. 2). Cranial 
flexure is more marked, and renders section of the entire brain 
area impossible. However, a series taken at a slight angle 
from the horizontal shows the ventral portion of the brain to 
be a simple tube, the walls of which are of uniform thickness 
and separated by a considerable lumen. In embryos of this 
age the neuromeric constrictions seem first to become promi- 
nent, though I find them also in stages about twenty-four 
hours younger. 
Fore-brain. 
As it was my object to study the mid-brain, almost all my 
sections were made with reference to that region. At first I 
had some difficulty in satisfying myself of its extent and loca- 
tion, the eyes being so large relatively that they were of no 
service as guides to this region. Later the determination of 
the anterior extremity of the medulla and the posterior com- 
missure perfectly defined its extent. The Cod brain is a long 
narrow area made up of closely placed small cells, round or 
spindle-shaped, with a clear undifferentiated border—the form- 
ing white brain substance. A short distance behind the axis 
of the eyes it is crossed by a broad band of white fibres, which, 
except for a few scattered cells, are continuous with the outer 
