NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 249 
the growth of the ear capsule. The place of origin of the roots of this 
nerve are variable. In swine and chick, for example, they have their 
origin from encephalomere VII, while in S. acanthias they arise behind 
this encephalomere. ‘This is correlated with the fact that the car cap- 
sule in S. acanthias is crowded backward into the region opposite enceph- 
alomere VII, whereas in the swine and chick the ear capsule continues to 
lie opposite encephalomere VI until long after the nerve assumes fibrillar 
connections with the neural tube. We may thus explain the variation 
in the position of the roots of this nerve, and still believe from the evi- 
dence that their primitive relations were with encephalomere VI. 
Again, the cells proliferated from encephalomere VII are those which 
pass into the fourth visceral (2d branchial) arch, and form the Anlage 
of the Urvagus, whose motor fibres innervate the musculature of that 
arch. The Urvagus assumes fibrillar connections with the neural tube 
at a point behind the origin of the glossopharyngeus, and the cause of 
this change of relation may safely be assumed to be the same as in the 
case of that nerve. We have good evidence, then, that the primitive 
relationships of four of the hindbrain neuromeres were with the first four 
visceral arches. ‘This relationship consists chiefly, but not wholly, in the 
fact that from these four neuromeres are proliferated cells which enter 
these arches and there form, in part at least,’ the ganglionic Anlagen of 
the nerves related with them. The origin of these cells from the neural 
crest would naturally lead us to infer that in dealing with them as nerve 
Anlagen we are not dealing with motor nerves. We are, however, 
really dealing with the Anlagen of nerves which later become mixed, 
But in later stages, when the nerve roots are established, the roots of 
only two of the nerves in question, viz. V and VII, have their exit from 
the encephalomeros from which their ganglionic Anlagen arose. Have 
we aright, then, to assume that the exits of the roots of the other two 
nerves, IX and X (Urvagus), have been pushed back from the position 
which may be assumed, on the evidence of the relations of their gan- 
elionie Anlagen, to have been the primitive one? I believe that we 
have, because, as we have seen from the examination of the relations of 
the roots of these two nerves, these roots lie as close to the point of 
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origin of their ganglionic Anlagen as the ear capsule will permit. In a 
1 Part of the neural-erest cells surrounds the mesoderm of the visceral arches, 
and very probably gives rise to some of the connective tissue of the arches. (See 
Plate 6, Fig. 40, el. ers. n.) Whether or not they later form the cartilages of the 
arches, as they are said to do in Necturus (Platt, 94,797), is a, question which 
requires more careful and prolonged study than I have been able to give. 
