456 
the horizontal muscle septum separating these two strands. In its 
course it has an anastomosing branch with the ramus recurrens 
facialis, which also lies between the two muscle strands. HATscHEX 
(92) finding that the ramus cutaneus dorsalis glosso- 
pharyngei “hinter dem ersten vorhandenen Myomer aufsteigt”, 
concluded on the basis of this evidence that “es ist keines der meta- 
otischen Myomere riickgebildet worden”. Although it is clear that the 
relation of this ramus cutaneus to the first myotome as stated 
by HATSCHER is a secondary one and therefore does not necessarily 
furnish evidence to warrant his conclusion, the facts stated above show 
this conclusion to be correct. 
I believe that Harscuex is correct in homologizing the M. rectus 
posterior oculi in Cyclostomes and Selachii. While because of 
want of material I have been unable to trace the development of this 
muscle in Petromyzon, I have no reason for doubting that it is 
derived from cells of the dorsal mesoderm as it is in Squalus. It 
seems to me highly improbable that muscles derived from different 
sources should win in the Vertebrate series the same peculiar relation 
to the eye. In the very early stages of development of the embryo, 
the cells of the dorsal mesoderm anterior to the otic invagination are 
scattered into a loose mesenchyma, the cells of which in part surround 
the eye. That these contribute to the formation of the connective 
tissue capsule of the eye and enter into the Anlage of the M. rectus 
posterior oculi I consider probable, although I am able to offer 
no evidence of the development of these mesenchymatous cells into 
muscle cells. Such a process is known to occur in Amniota. Assuming 
then the exact homology of the rectus posterior myomere in 
Petromyzon and Squalus, and also, on the basis of the evidence above 
stated, of the first post-otic somites in these two forms, we may tabulate 
the metamerism of nerves and somites in the following way (p. 457): 
From an examination of this table the following conclusions may 
be drawn: The two anterior myotomes of those which in Petromyzon 
form the M. par. subbranchialis are the exact homologues of 
the posterior two of those myotomes which in Squalus form the 
hypoglossus musculature. The group of myotomes which form the 
M. par. subbranchialis of the one is not exactly homologous 
with the group of myotomes which form the hypoglossus musculature 
of the other. The exact homologues of myotomes which in Squalus 
send buds posteriorly into the pectoral plate, in Petromyzon send 
buds anteriorly into the branchial region. That myotome (7th post- 
otic) which in Petromyzon forms the anterior portion of the M. 
