214 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



close to the ectoderm. It is evident that the neural-crest cells migrate 

 around the most expanded portion of the vesicle, so that they come to 

 lie in the regions of constriction anterior and posterior to the dilated 

 vesicle. They migrate, as it were, into the spaces where there is room 

 for them. The cells of these two portions are in continuity dorsally, as 

 in the previous stage. As a result of the expansion of the dorsal wall of 

 the neural tube in the region of encephalomere III, the cells of the 

 neural crest are laterally displaced in this region, so that they appear in 

 optical sagittal section (Fig. 9) to have taken a more ventral position. 

 Mitrophanow ('93) has given the name " le groupe nerveux anterieur" 

 to the anterior smaller portion of the trigeminus Anlage, and states 

 that " dans la plupart des cas, ce groupe' est pen separe " (i. e. from the 

 "groupe du nerf trijumeau"). Coggi ('95) finds that in Torpedo this 

 anterior portion of the trigeminus Anlage arises as a paired structure, 

 the lateral halves of which secondarily unite in the mid-dorsal line ; 

 Coggi, however, agrees with Mitrophanow that this anterior part of the 

 trigeminus is at first distinct from the posterior larger portion. In 

 S. acanthias, however, I find that both anterior and posterior parts 

 form at first a continuous neural ridge, which lies dorsal to the midbrain 

 vesicle. Only in later stages does the anterior portion become separated 

 as the so called thalamic nerve. At the stage with 19 or 20 somites the 

 cells proliferated from encephalomere V extend somewhat farther ven- 

 trad toward the hyoid arch than in the preceding stage, and at the same 

 time a proliferation of cells from the mesoderm extends dorsad to meet 

 them. The mesodermal cells migrate from both sides of the constriction 

 between van Wijhe's second and third somites, and fx'om them extends a 

 cellular process toward the ganglionic Anlage.-^ 



The conditions remain practically unchanged in a stage with 21 or 22 

 somites (Plate 3, Fig. 10). The anterior and posterior portions of the 

 trigeminus Anlage now extend into the region ventral to the midbrain 

 vesicle, and are about to unite with each other. The cells in the region 

 of encephalomere III have undergone a still greater lateral displace- 

 ment, from which one may infer that cells are no longer proliferated 

 from the neural crest of this encephalomere. It is seen that the cells of 

 the acustico-facialis are now united with the cellular process from the 



1 I have been unable to determine that these noesodermic cells participate in the 

 formation of the Anlage of the nerve. It appears to me, however, that such a re- 

 sponse on the part of the somites to the development of a nerve Anlage is a fact 

 which cannot be ignored in dealing with the question of nerve development. See 

 also similar evidence in the description of the development of the trochlearis and 

 oculoraotorius. 



