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to the ear in the form of two buds one lying above the other. In 
this forward extension of the first somite, which is now differentiated 
as the first myotome, cells of the mesenchyma lying anterior to the 
otic capsule take no part. The changes which occur in later stages 
consist simply in the greater forward extension of the dorsal and 
ventral strands of the first myotome toward the snout and in a con- 
comitant reaching forward of the following myotomes (see Fig. 2). 
The median portions of the first two post-otic somites degenerate, as 
has been stated by Kuprrer ('94—’96), in later stages. 
The following considerations lead me to believe that the first 
post-otic somites in Petromyzon and Squalus are exactly homologous: 
1) In both forms, the somite in question (VAN WiuHe’s 5th in Squalus) 
is connected in a similar way with the third visceral arch. It is 
particularly to be noted that such a relation of a somite with a given 
visceral arch is not a purely topographical one. 2) In both forms, 
this somite develops a myotome (embryonic in Squalus); the somite 
anterior is crushed into loose mesenchyma by the development of the 
otic capsule; the somite posterior is cut through longitudinally during 
development by the vagus (X). 3) In both forms the ganglionic Anlage 
of the glossopharyngeus fuses with the skin at a point opposite the 
constriction separating the somite in question from the one anterior, 
and the ganglionic Anlage of the vagus, at least its anterior portion, 
fuses with the skin at a point opposite its posterior constriction. 4) The 
conclusion based on the evidence of topographical relations in early 
stages of development to the otic capsule, third visceral arch and the 
Anlagen of the cranial nerves, is, furthermore, confirmed by the relations 
of the rami cutanei dorsales of the IX, X and 1st spinal nerves. 
It is well known, especially through the recent remarkable researches 
of Kuprrer, that the nervous system of Petromyzon is very primitive. 
This is true both of cranial and spinal nerves. While in the Myxinoids 
dorsal and ventral spinal nerve roots unite median to the somites as 
in higher Vertebrates, in Petromyzon dorsal and ventral nerve roots 
remain separate as in Amphioxus (ScHNEIDER), and the dorsal roots 
pass as in that form between the myotomes to the skin. Each dorsal 
root lies in the myoseptum anterior to its proper myotome (GOETTE, 
SCHNEIDER, VAN WIJHE, HATSCHER). The first myotome in Petromyzon 
whose relations to dorsal and ventral spinal nerve roots are typical 
is the 4th post-otic myotome (see Fig. 2). Its dorsal nerve root is 
that spinal nerve to which Harscnex (92) has given the name 
“spinalartige Vagusanhang”, because it unites in the later stages of 
development with the vagus. As has been stated by HatscneK (92) 
