PRIMITIVE SEGMENTATION OF THE VERTEBRATE BRAIN. 465 
neuromere, thus making the fore-brain contain three neuro- 
meres (see figs. 6 and 7, . 3). I think that this conclusion 
is strengthened by the fact that both Orr and McClure assigned 
to the fore- and mid-brain together five neuromeres, though 
they did not definitely determine the number composing each ; 
and I feel sure that investigation will confirm my observations 
on this point. 
Mid-brain. 
Orr has stated that this region of the brain appears equal 
in length to two hind brain neuromeres. McClure has made 
it even longer; but I think he is mistaken in assigning to it, 
as he does, though on purely speculative grounds, a third 
neuromere. I think it most probable that accurate measure- 
ments would make it slightly longer than Orr has estimated 
it; for while I find it contains but two neuromeres, I have 
also found that the segments increase in length toward the 
fore-brain, those of the spinal cord, the myelomeres, being 
uniform in size, but smaller than the encephalomeres, the 
largest of which is the first, or olfactory. This fact may be 
accounted for perhaps on phyletic grounds by the rapid de- 
velopment of the forward portion of the brain. Sections of 
Cod of about eleven or twelve‘days, the plane of section having 
reached almost to the floor of the canalis centralis, exhibit a 
region extending from the posterior commissure to a point 
some little distance behind the eyes (see fig. 3). Within these 
limits there appear two well-marked convolutions of the brain- 
wall. Owing to the cranial flexure at this stage it is difficult 
to obtain true longitudinal sections, but by comparison with 
those of younger stages I have fairly well identified this region 
as the mid-brain. The constrictions are slightly smaller than 
those of the fore-brain, and rather more semicircular in shape. 
The characteristic radial arrangement of cells is present, but 
I have not been able to satisfy myself with regard to their 
relation to nerves. The first one, at a low level, seems to give 
origin to some fibres which may correspond to the third nerve ; 
from the second I have observed no nerve originating. I am 
