NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 218 
Trijumeau ” (Mitrophanow, ’93), and “ erste periaxiale Strang ” (Gorono- 
witsch, '93). Its cells at this stage (15 or 16 somites) have already 
migrated half way down the side of the neural tube (Fig. 7). In the 
region of encephalomere V the disassociation of the cells of the neural 
crest has begun, and the dorsal part of the encephalomere in consequence 
appears enlarged. A ventral migration of its cells, however, does not 
take place until a later stage. 
In an embryo of 18 or 19 somites (Plate 3, Fig. 8) two regions of cell 
proliferation, separated sharply by encephalomere IV, are seen. Mi- 
trophanow (93) has stated that at the beginning the facialis is not 
wholly separated from either the trigeminus or the vagus group. I find 
on the contrary, as already stated, that no neural crest is found in the 
region of encephalomere IV, and that consequently the “ Trigeminus 
Anlage” is separated by the space of this encephalomere from the pos- 
terior portion of the neural crest. Apparently as a consequence of cell 
proliferation and migration, the dorsal wall of encephalomere III is very 
thin at this stage, while that of encephalomere IV is considerably thicker 
and its cells are more compactly arranged. The cells of the neural ridge 
which form the “ Trigeminus Anlage ” now extend ventrally as far as van 
Wijhe’s second somite. The second region of cell migration is at this 
stage sharply confined to encephalomere V. Behind this a disassocia- 
tion of neural-crest cells has begun in the region of encephalomere VI, 
but no migration has taken place. From an examination of later stages, 
the cells proliferated from the region of encephalomere V are easily 
proved to pass ventrally into the hyoid arch, and to form the gangli- 
onic Anlage of the acustico-facialis. From a study of mitotic cells and 
from the grouping of cells one is led to believe that the greatest cell 
proliferation takes place in the posterior part of this neuromere. 
It is to be noticed that the advancing ventral end of the ganglionic 
Anlage extends toward the cleft between van Wijhe’s third and fourth 
somite. Also that cell processes from each of these somites now extend 
toward the ganglionic Anlage. 
When the embryo possesses 19 or 20 somites (Plate 3, Fig. 9) the 
“Trigeminus Anlage” shows a differentiation into an anterior smaller 
portion, which passes in front of the midbrain vesicle toward the optic 
evagination, and a posterior larger portion, which extends ventrally into 
the mandibular arch, just beneath the superficial ectoderm and external 
to the second somite. I am inclined to believe that this division of the 
Anlage is partly due to the enlargement of the vesicle of the midbrain, 
since frontal sections show that the lateral wall of the midbrain lies very 
