NEAL : NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 215 



mesoderm.^ This process may be traced dorsallj 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 venti'ally 

 below tlie lateral midbj-ain 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. Tlie 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, its position being clearly 

 inter-somitic. Posteriorly, in the region of encephalomere VI, and to a 

 considerable extent behind 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 Y 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 Vf. Both 

 Rabl ('92) and Hoft'mann ('9-1) 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, Dohm ('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, witli the exception 

 of Herrick and Broman (see Table II., p. 152), have stated that the glosso- 

 pharyngeus is related to hindbraiu neuromere VII, ^ it seems well to call 

 attention to the fact that the cells of the ganglionic Anlage of this nerve 



1 Is 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 Piatt ('89) stated that the glossopharyngeus is connected with the pos- 

 terior constriction of encephalomere VI. 



