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before the edges have fused dorsally to form a closed tube, shows 
that the neural crest is already differentiated from the tissue which 
will form the neural tube; it is differentiated as a region of rapid cell 
proliferation and of less compactly arranged nuclei. 
My interpretation, then, differs from Locy’s, since he finds the 
neural ridges segmented regularly, and considers the “segments” as 
survivals of an ancestral segmentation, whereas I find the edges of 
the neural plate irregularly and somewhat transitorily segmented, the 
irregularity and inconstancy of the segments precluding, in my opinion, 
a phylogenetic interpretation. To demonstrate that Locy has not 
accurately traced the “neural segments onward in unbroken conti- 
nuity until they become the neuromeres of other observers”, I pro- 
pose to discuss the relation of the “neuromeres” to the posterior 
limit of the cephalic plate. 
2. Limits of cephalic plate. 
Locy has called attention to the fact that in the earlier stages 
of the embryo, before the neural plate has formed a closed tube, head 
and trunk may be distinguished. “It is possible”, he says (94, p. 543), 
“in very young stages to draw a line indicating where the expanded 
part of cephalic plate joins the non-expanded part of the embryo..... 
This is, the in Squalus acanthias, just in front of the point where, sub- 
sequently, the vagus nerve begins. .... In this animal, we may 
identify that part of the head that lies in front of the vagus nerve 
by counting the first eleven neural segments. It will be merely a 
question of agreeing upon the number of primitive segments belonging 
to the vagus, to enable us to locate with definiteness the hindermost 
limit of the head. Besides being of use in other ways, this would 
enable us to say, even in the earliest stages, what is head mesoblast 
and what is trunk mesoblast”. 
I must confess that I cannot see that Locy’s determination of 
the limits of the cephalic plate helps us in determining the boundary 
of head and trunk. This boundary, as he states, has still to be 
determined. To fix the limit of the head mesoderm by a direct 
study of the mesoderm itself is quite as easy as to determine its 
boundary by the still hypothetical posterior limit of the vagus. The 
posterior limit of the cephalic plate as determined by Locy actually 
separates neither what is pre-otic from what is post-otic, nor head 
from trunk. 
My own observations differ fundamentally from those of Locy, 
since they show that the line which separates the expanded cephalic 
