NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS, 217 
beginning opposite encephalomere VI. In parasagittal sections the 
Anlage of the glossopharyngeus appears clearly distinct from that of the 
vagus, while in the median plane they are seen to be continuous portions 
of the neural crest. 
When the embryo has 28 to 30 somites (Fig. 15) the conditions, so 
far as the trigeminus is concerned, are practically unchanged. Neural- 
crest cells still persist in the regions, of constriction between the primary 
brain vesicles. Thus, three strands of neural-crest cells are seen to lie 
in the region of constriction between the brain vesicles, just as they do 
in the trunk between the myelomeres. The ganglionic Anlage of the 
acustico-facialis, which had fused with the thickened auditory epithelium 
in the early stages of its development, now, as the nerve Anlage recedes 
from the ectoderm, retains this connection, forming thus the Anlage of 
the acusticus, The acusticus therefore in its development and relations 
resembles a ramus dorsalis of a cranial nerve, 
The cells of the glossopharyngeus have been further displaced. In all 
the specimens of this stage which I have examined, two distal portions 
of the nerve Anlage may be distinguished. The fate of the posterior of 
these is unknown to me. The cells of the anterior portion pass ventrally 
into the third visceral arch, and are related to the constriction between 
van Wijhe’s somites 4 and 5. In precisely the same way the Anlage 
of the seventh nerve occupies the cleft between the third and fourth 
somites. The advancing ganglionic Anlagen pass close to the superficial 
ectoderm in the plane of the constrictions between the somites. Simi- 
larly the Urvagus Anlage meets the mesoderm at the posterior cleft of 
the fifth somite. This fact seems to me to be of some importance in 
considering the question whether the branchial nerves are somitic or in- 
tersomitic in position, and to warrant the conclusion that the cranial 
nerves resemble the dorsal nerves of Amphioxus in being intersomitic, 
as well as in other respects. Ata stage with 33 or 34 somites (Plate 3, 
Fig. 14) the trigeminus Anlage retains connection with the mid-dorsal 
line of the neural tube in only two restricted regions, anteriorly by the 
“thalamic” portion, and posteriorly (in the region of the constriction be- 
tween midbrain and hindbrain) by a strand of cells to which Miss Platt 
has given the name “primary trochlearis.” Posteriorly the cells of the 
trigeminus Anlage are grouped into a somewhat thickened mass opposite 
the posterior part of encephalomere III, the first indication of the differ- 
entiation of the Gasserian ganglion. The Anlagen of the acustico-facialis 
and the glossopharyngeus have become farther separated by the invagina- 
tion of the auditory epithelium, the displacement affecting the cells of 
