172 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
neuromeres never shift their position, the only change being a gradual 
assumption, by each of the local thickenings, of an inner concavity in 
the region of the lateral zones. 
Frontal sections of an embryo 15 mm. long show that dorsally all 
traces of the neuromeres are lost. A frontal section in the region of 
the lateral zones from an embryo of this stage is represented in Figure 
30, Plate 5. A great separation of the lateral walls of the medulla is 
seen to have taken place in the region of neuromeres III, IV, and V. 
At this stage only do the neuromeres possess the characteristics de- 
scribed by Orr for the Lizard (see page 167). While the external con- 
strictions are only faintly shown, owing to the increase of the “ white 
substance ” on the sides of the medulla, the internal ridges and con- 
cavities are well marked. From this stage onward the neuromeres 
begin to disappear. In embryos of 40 mm. to 50 mm., neuromere VI, 
in relation with the facialis nerve, is the most clearly marked of the 
neuromeres, 
Before passing to an examination of the evidence of neuromeres in the 
trunk region, I wish to emphasize the fact that the hindbrain neuro- 
meres cannot be regarded as structures dependent upon the pressure of 
mesodermal somites. “Being local thickenings of the lateral wall of the 
neural tube they are obviously inexplicable on such simple mechanical 
basis. They are structural differentiations of the tube in regions where 
the mesoderm has not yet extended, — that is, in the dorsal and lateral 
portions of the tube, the mesoderm of the head being still ventral in re- 
lation to the neural tube. 
C. SUMMARY. 
In the preceding study of the hindbrain neuromeres in $. acanthias, 
I have supplemented Orr’s criteria (applicable to later stages) by a de- 
scription of the structure of the neuromeres in Squalus in earlier stages 
of development, i. e. in embryos of 14-50 somites. The characteristics 
possessed by hindbrain neuromeres in these earlier stages may be sum- 
marized as follows. Each neuromere is separated from its neighbor by 
an external constriction, which passes entirely around the neural tube. 
There is dorsally and ventrally an internal ridge corresponding to this 
external constriction ; but the ridge vanishes in the region of the lateral 
zones, being replaced by an internal depression or groove. The nuclei 
of the lateral wall are, however, still arranged (Fig. 37) in a manner 
which approximates that of the region of the internal ridges, notwith- 
standing that the thickening of the lateral wall of the neuromere has 
