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 peu séparé” (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 from 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 T have been unable to determine that these mesodermic 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 
oculomotorius. 
