Kingsbury, Bi-ain of Necturus. 167 



In the region of the second spinal nerve (non-ganghon- 

 ated), the myehnic fibers of the dorsal columns have begun to 

 decrease in number, almost all having disappeared save such as 

 will constitute the ascending tracts of the fifth and tenth nerves, 

 whose fibers begin to be recognizable at this level. From this 

 point to the beginning of the metatela the change consists 

 merely in a migration laterad of these tracts and the dorsal ex- 

 tension of the myelocoele. In the ventral regions of the myel 

 changes are less marked and consist merely in a more compact 

 arrangement of the fibers of the posterior longitudinal fasciculus. 

 In the relations of the cell areas, the change from myel to ob- 

 longata is not so perceptible. A general view of the distribu- 

 tion of the cells in the oblongata may be given here. As the 

 oblongata widens the general arrangement of cells remains 

 much the same. The large cells of the ventral region are con- 

 tinued cephalad for some distance, but become more scattered 

 beyond the tenth nerve ; such cells are found, how- 

 ever, at intervals throughout the length of the oblongata, 

 and in the region of the eighth nerves they are increased in 

 number and size. The lateral cells of the myel become sup- 

 planted in posision by the motor nidi of the tenth, ninth and 

 seventh nerves, which are not so distinctly demarcated in Nec- 

 turus as in Cryptobranchus (Osborn, '88). Farther cephalad, 

 the compact motor nidus of the fifth nerve succeeds in the 

 latero-ventral region. The cells occupying the remainder of 

 the oblongata, except large, laterally situated cells near the 

 eighth nerve and similar cells more sparsely located, are small, 

 — so called sensory cells — especially occupying the dorso-lateral 

 cinerea and the region on each side of the meson, where be- 



name in Petromyzon. If this were correct, — and it would seem but partially so, 

 — no recognition of a distinction between the posterior longitudinal fasciculus 

 and Miillerian fibers should be made, since in Petromyzon two or three Miillerian 

 fibers continue cephalad as the axis of the posterior longitudinal fasciculus (in 

 the restricted sense of Ahlborn). This bundle will be recognized here occupying 

 its usual place just ventrad of the cinerea on each side of the meson, and many of 

 the large fibers composing it seem to be comparable in their relations with the 

 Miillerian fibers of the Lamprey. 



