NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 211 
and it is certainly not true for Squalus, and not true, so far as I am 
able to determine, for Amblystoma. In Petromyzon, however, as has 
been previously stated by Scott (’87) and Shipley (’87), the spinal gan- 
glia lie opposite the constrictions between the somites (in later stages 
opposite the myosepta)." Thus, inasmuch as the dorsal nerves of Ammo- 
costes are ¢ntersomitic and never unite with the ventral nerves which 
are somitic in position, and inasmuch as the dorsal ganglia show close con- 
nection with the ectoderm in early stages of development and lose this 
connection during development, the spinal nerves of this animal form 
a natural transition from the nerves of Amphioxus to those of Squalus 
and higher Vertebrates, For in Amphioxus ventral nerves are somitic 
in position, dorsal nerves intersomitic, and the connection of the ganglia 
of the latter with the skin is retained throughout life.” Two chief causes 
seem to have brought about the change in the relations of the dorsal 
spinal nerves in the Vertebrate series. The first cause appears to have 
been the great dorsal and anterior extension of the trunk myotomes, and 
the second cause the posterior extension of the ramus cutaneus dorsalis 
vagi (ramus lateralis vagi), which takes the place of the rami cutanei of 
the spinal nerves. The physiological reason for the extension of the 
vagus is to be found in the advantage obtained from the centralization 
of sensory impulses in the brain. With van Wijhe ('92), Hatschek (’93), 
and M. Fürbringer (97), I accept the theory of Prochaska, Sómmering, 
and Gegenbaur that cranial and spinal nerves are homodynamic, and the 
view of Hatschek (’92) that dorsal and ventral nerves primitively alter- 
nated with each other? Of these, the former were mixed in function and 
the latter motor, as in Amphioxus. 
c. Nerve RELATIONS IN THE CEPHALIC REGION or S. ACANTHIAS. 
In the head, where the nerve relations are much more complicated, it 
will be necessary to trace tho development of the nerves in different 
stages. The series represented in Figures 7 to 21 (Plates 3 and 4) is 
intended to show the changes which the neural crest (colored in blue) 
undergoes, and likewise to show the development of the brain vesicles 
1 Because of this relation to the myomeric constrictions in Ammocates and the 
relation of the ganglia to the expansions of the spinal cord (myelomeres) deducible 
from it, it is obvious that not very great morphological value can be given to the 
fact that in Squalus the ganglia lie opposite the constrictions of the spinal cords. 
2 I hold with Hatschek (’92) and M. Fürbringer (’97) that in Amphioxus the 
homologues of the dorsal ganglia of Craniota are found in the cell groups at the 
place where the dorsal nerves meet the skin. 
8 See also Ransom and Thompson ('86). 
