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 1 1), 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 10 (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 segments," 

 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 VII is differentiated from 

 the region of the neural tube which lies behind the broad cephalic 

 plate, and does not become clearly marked ofi" from the spinal cord 

 region before a considerably later stage (stage H of Balfour). There- 

 fore, if Locy's neural segment 1 1 is identical in position with my en- 



