NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 197 
The cavity of the somite is enlarged ventrally opposite that portion of 
its median surface where a rapid proliferation and migration of cells 
appears to take place. I see no reason why the more dorsal and lateral 
portion of the mesoderm should not be homologized with the myotome 
portion, and the ventral median region with the sclerotome portion of 
trunk somites. Iam unable to detect any essential difference between 
the phenomena presented in this section and those presented in sec- 
tions made in the region of van Wijhe’s 5th and 6th somites, to which 
Rabl grants “ Bürgerrecht” as true somites. The greater dorsal extent 
of the latter cannot be regarded as an essential difference. Here, as 
there, we find a well marked and definite region of cell proliferation. 
As development goes on, the cavity of the 3d somite increases in volume, 
and at the same time the somite grows forward, chiefly by the elongation 
of its anterior end, median'to the Gasserian ganglion. In confirmation 
of the statement of Miss Platt (91), I find that the first muscle cells 
are differentiated in the median wall of that portion of the somite which 
at this stage lies posterior 
to the Gasserian ganglion. 
The great extension, of the 
anterior portion seems to 
retard its histological dif- 
ferentiation. But in this 
portion also, when muscle 
colls appear, they are found 
in the median wall. Rabl 
(89, p. 236) says: “ Wäh- 
rend ferner die Muskulatur 
der Urwirbel ausschliesslich 
aus der medialen Wand ent- 
steht, nimmt sie im Vorderkopf zum grössten Theil aus der lateralen und 
zum kleineren Theil aus der hinteren Wand der sogenannten Somite: den 
Ursprung.” A cross section of the myotome of the 3d somite at a late 
stage of development appears to me to refute this statement (Figure B). 
It is clear from an examination of the phenomena presented in such a 
Ficure B. 
Wijhe’s 3d somite, in a late stage of development (20 mm.). Elongated muscle 
cells are already differentiated in the median wall (muscle plate), while the lateral 
wall (cutis plate) retains its epithelial character. X 50. 
a, cell migrating from the median wall of the myotome into the myocal; 
gn. Gas., Gasserian ganglion; la, ct., cutis plate; la, mu., muscle plate; my’cal., 
myoceel. 
