NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 229 
wand immer noch unmittelbar an, dies ist auch jetzt noch grösstentheils 
der Fall, aber mit ihrem medialen Rand fängt sie jetzt an sich von der 
Gehirnwand zurückzuziehen, bleibt aber mit ihr durch einen kurzen, 
dicken Zellstrang kontinuirlich verbunden. Dieser Zellstrang bildet die 
Anlage des Nervus olfactorius, aber es ist nicht möglich zu sagen, wel- 
chen Antheil die Epidermis und welchen das Gehirn an der Anlage der 
Riechnerven nimmt, denn er entsteht aus dem letzten Rest des kontinu- 
irlichen Zusammenhanges von Epidermis und Medullarwand, welcher 
von Anfang an bestanden hat.” 
His (89°) had previously found in the human embryo that the first 
step in the formation of the olfactory nerve was the migration of mesen- 
chymatous cells between the olfactory plate and wall of the brain, 
Later the olfactory ganglion is formed by the migration of cells from the 
lateral walls of the olfactory epithelium. Finally, the olfactory nerve 
results from the assumption by these cells of a bipolar form and the 
elongation of the poles both centripetally and centrifugally to form 
fibrillar connection with brain and olfactory pits. 
My own observations concerning the development of the olfactorius are 
as yet incomplete, and I am not able to add much to the evidence which 
has been given, In agreement with Hoffmann (96) I find that, as the 
olfactory plate and the brain wall separate, they retain connection with 
each other by faintly staining fibrils in the region of the future olfactory 
pits. Whether these fibrils enter into the formation of the definitive 
olfactorius I am not able to state, and the observations of Hoffmann 
appear to me insufficient to establish this fact. My results and those 
of Hoffmann do not agree; for he finds in embryos of 16 mm., and still 
more clearly in embryos of 18-20 mm., that mesenchymatous tissue 
“schiebt sich von allen Seiten zwischen Medullarwand und basale Nasen- 
grubewand ein, und in demselben Grade als beide sich entfernen, nimmt 
natürlich der Riechnerv an Länge zu.” I infer this mesenchymatous tissue 
to be the same as that which Hoffmann previously states to be dertved 
from the “anterior head cavities.” My observations, however, lead me 
to agree with Marshall (’78), that the cells which appear between the nasal 
pit and the brain wall, as these separate, are neural-crest cells. Van 
Wijhe may be technically correct in stating that the neural crest has 
disappeared in the region of the forebrain at the time when the olfactory 
nerve is established ; but it is certainly not true that the neural-crest 
cells in the region of the’ forebrain have done so at this stage. They 
persist in the region of the forebrain which lies opposite and anterior to 
the optic vesicle, aud in my opinion are thé tells which migrate between 
