NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 165 
the posterior boundary of somite 7 to the posterior boundary of neuromere 
VI, after the closure of the neural tube (see Figs. 7 and 10, Plate 3, and 
Fig. 6, Plate 2), and, as previously stated, the posterior boundary of the 
auditory invagination at first coincides with the posterior boundary of 
encephalomere VI. Again, and in direct confirmation of the evidence 
stated above, the posterior boundary of encephalomere VI is the pos- 
terior boundary of a greatly enlarged portion of the neural tube (Figs. 
7-10, Plate 3), as one would naturally expect, if it coincides with the 
posterior boundary of the previously widely expanded cephalic plate. 
With this fact in mind it is interesting to compare the conditions I have 
found with Locy’s results. I believe he would not contest the assertion 
that my encephalomere VI is identical with his neuromere 10 (Zimmer- 
mann’s encephalomere 11), because its relation to the ear vesicle at the 
time this is formed makes its identification a simple matter. Locy (’95, 
p. 522) says of the auditory vesicle: “When first established its centre 
occupies the space of the segment marked 10. Sometimes, in its earliest 
stages, the circular area spreads over the space of the three segments 
marked 9, 10, and 11, but I should say from my observations that, more 
frequently, it is not so widely expanded. It always settles down in 
Squalus acanthias to occupy the position first indicated, and subsequently 
it is shifted backwards.” This accords with my identification of his seg- 
ment 10 with my encephalomere VI, and this conclusion is corroborated 
by his statement that “the segment marked 8 is seated above a depressed 
region in which the first visceral cleft appears,” for that is precisely the 
position of the encephalomere IV of my figures. On page 528, however, 
he says, “When the ear vesicle first arises it makes its appearance 
opposite the ninth neuromere ” (!). Again, in his Figures 6 and 9, Plate 
XXIX., “neural segments,” which are described (p. 528) as 8 and 9, but 
which I believe to be segments 9 and 70 (as a comparison with my 
Fig. 46, Plate 7, shows), are numbered 7 and 8(!). Here, then, are 
three conflicts. Despite the elusive nature of Locy’s “neural segmerits,” 
I am disposed to regard his neural segment 10 (opposite which, as he 
has twice stated, the auditory invagination occurs) as identical in posi- 
tion with encephalomere VI of my figures. If this be true, there is no 
room on the cephalic plate for his neural segment marked 11, since, 
according to my determination, encephalomere VIT is differentiated from 
the region of the neural tube which lies behind the broad cephalic 
plate, and does not become clearly marked off from the spinal cord 
region before a considerably later stage (stage H of Balfour). There- 
fore, if Locy’s neural segment 11 is identical in position with my en- 
