162 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
regard such irregularities of the edge of a rapidly expanding plate of 
tissue as of morphological importance. It is very significant that the 
segments show most prominently in the cephalic-plate region just before 
the edges of the plate begin to rise dorsally, for it is likewise at this 
stage that I find the first evidence of the disassociation of cells along 
the edges of the neural crest. Such a disassociation of cells, or even a 
rapid proliferation of cells, — which certainly does occur in this region, 
— would lead to such phenomena as those reproduced in Figures 1 and 
2, Plate 1. An examination of cross sections of the cephalic plate 
(Plate 7, Figs. 55 and 56) 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 walls of the neural tube; it is differen- 
tiated as a region of rapid cell’ proliferation and of less compactly 
arranged nuclei. If the centres of cell proliferation were fixed, then 
we should have a segmented neural ridge, as affirmed by Beard (’88). 
My interpretation differs from Locy’s, since he finds the ‘neural 
ridges” segmented regularly, and considers the segments as sur- 
vivals 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. Locy’s results from surface studies seem 
to me to be a confirmation of those reached by Beard (88), who, in 
studying the development of the peripheral nervous system in Selachii, 
found from the examination of sections that the neural crest is differen- 
tiated before somites appear, and that it is from the beginning segmented. 
Beard’s conclusions have, however, never been confirmed, and have in- 
deed been regarded by Dohrn (’90, p. 55) as quite untenable. 
To demonstrate that Locy has not accurately traced the “neural seg- 
ments” onward in unbroken continuity until they become the “ neuro- 
meres of other observers,” I propose to discuss the relation of the 
neuromeres to the posterior limit of the cephalic plate. 
f. Limit or “OBPHALIO PLATE.” 
Locy (’95, p. 543) has stated that in early stages of the embryo, before 
the neural plate has formed a closed tube, head and trunk may be distin- 
guished, “It is possible,” he says, ‘in very young stages to draw a line 
indicating where the expanded part of the cephalic plate joins the non- 
expanded part of the embryo. . . . This is, in Squalus acanthias, just in 
front of the point where, subsequently, the vagus nerve begins. . . . In 
this animal, we may identify that part of the head which lies in front of 
