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 swiue 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 ear cap- 

 sule in S. acanthias is crowded backward into the region opposite enceph- 

 alomere VII, whereas in the swiue and chick the ear capsule continues to 

 lie opposite encephalomere A'l 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 ui the 

 case of that nerve. We have good evidence, then, that the primitive 

 relationships of four of the hindhrain 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 gangiionic 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 encephalomeres from which thpir ganglionic Anlagen arose. Have 

 we a right, 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- 

 ghonic 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 

 origm of their ganglionic Anlagen as the ear capsule will permit. In a 



1 Part of tlie neural-crest 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, d. crs. n.) Whether or not they later form the cartilages of the 

 arches, as they are said to do in Necturus (Piatt, '94, '97), is a question which 

 requires more careful and prolonged study than I have been able to give. 



