NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 245 
and spinal cord of snake embryos not explicable on mechanical grounds. 
“The neuromeres of the medulla cannot be ascribed to the mechanical 
influence of the Anlagen of the nerves, for those segments which have 
no nerves develop equally with the others” (cf. Froriep, *91). He 
considers however “the neuromeres of the forebrain” region wholly 
illusory from a morphological standpoint, since they involve only dorsal 
structures. 
C. INTERPRETATION OF HINDBRAIN NEUROMERES IN SQUALUS ACANTIITAS. 
I believe that the evidence which I have obtained from a study of the 
development of hindbrain ‘neuromeres in 8. acanthias excludes the 
possibility of a simple mechanical explanation of them. In their earlier 
stages they were seen to be local thickenings of the lateral walls, a 
phenomenon intelligible only on the ground of unequal growth, and not 
in the least explicable as the result of the passive bending or shoving 
of a tube already formed.* Since the somites do not extend into the 
region of the dorsal part of the encephalomeres, the possibility that the 
neural tube in this region is constricted by them is excluded. They are, 
then, in both structure and mode of development, clearly not to be ex- 
plained in the same way as the myelomeres. Again, that they are not 
due to the effect of the Anlagen of the nerves, as supposed by Froriep, 
is shown by the fact that encephalomere LV develops equally with the 
others, although there is no nerve in relation with it until a compara- 
tively late stage. Since the fibrillar connection of nerves with neuro- 
meres is established almost at the same time that the inner surface of 
the hindbrain neuromeres becomes concave, it might be thought that this 
change is due to the mechanical effect of nerve fibres. That such is not 
the case seems clear, however, because no nerve fibres come into relation 
with the outer convexity of encephalomere VI. The hindbrain neuro- 
meres, from their early appearance onwards until they disappear, are 
local differentations of the walls of the medulla, and as such are not, I 
believe, to be satisfactorily explained on simple mechanical grounds. On 
the other hand, I hold that they do possess certain characteristics which 
admit of a mechanical explanation. This seems to be supported by evi- 
dence from two sources. In the first place, a fixing agent which causes 
a contraction of the tissues of the embryo intensifies the constrictions 
between the neuromeres. By this means the radial arrangement of cells 
1 This is true also in swine and chick embryos; but I do not find in Ambly- 
stoma as good evidence that the neuromeres are local thickenings of the neural 
wall. 
