NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 215 
mesoderm.’ This process may be traced dorsally to a point outside of 
the ganglionic Anlage, i. e. between it and the superficial ectoderm. 
The future course of the nerve is along the line of the process. Between 
this and the next succeeding stage, which is represented in Figure 11, 
the trigeminus Anlage undergoes a considerable change. The anterior 
(thalamic) and posterior (trigeminal) portions having fused ventrally 
below the lateral midbrain swelling, now extend ventrad as a continuous 
sheet with two ventral processes, one reaching into the mandibular arch 
and the other to a point below the eye vesicle. The anterior (thalamic) 
portion has assumed a more compact appearance, and extends from the 
region of the constriction between forebrain and midbrain, both ventrad, 
to a point above and behind the eye vesicle, — where, as already stated, 
it meets the anterior prolongation of the trigeminus portion, —and an- 
teriad to a point in front (dorsad) of the eye vesicle. The acustico- 
facialis Anlage now extends into the hyoid arch, ¿ts position being clearly 
inter-somitie. Posteriorly, in the region of encephalomere VI, and to a 
considerable extent bebind this, the cells of the neural crest have begun 
their ventral migration. At this time, then, a continuous neural ridge 
or crest extends from the anterior boundary of encephalomere V back- 
ward into the region of the spinal cord. In cleared specimens and in 
parasagittal sections the neural crest cells seem discontinuous in the 
region of constrictions between encephalomeres IV, V, and VI. Both 
Rabl (92) and Hoffmann (’94) have held that the pre-auditory portion 
of the neural crest is discontinuous with the post-auditory portion, and 
Rabl considers this another proof that the pre-auditory region is one 
“sui generis.” On the other hand, Dohrn (90) and Mitrophanow (93 
have stated, like the present author, that they find the crest continuous 
in the two regions, 
A well marked proliferation of cells seems to take place in the region 
of encephalomere VI. These cells may be traced continuously into later 
stages, until they enter the first branchial arch and form the Anlage of 
the glossopharyngeus. Since previous investigators, with the exception 
of Herrick and Broman (see Table IT., p. 152), have stated that the glosso- 
pharyngeus is related to hindbrain neuromere VIT? it seems well to call 
attention to the fact that the cells of the ganglionic Anlage of this nerve 
1 Ts this mesodermal process the median branch of Kupffer’s typical segmental 
nerve? Its relation to the mesoderm leads me to believe that this is the case. It 
soon disappears, as stated by Kupffer (91). 
2 Miss Platt (’89) stated that the glossopharyngeus is connected with the pos- 
terior constriction of encephalomere VI. 
