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evident that we must choose one whose relations are of such a 
character that the assumption of an exact homology will he the least 
doubtful possible. HATSCHEK (’92), who alone has attempted to 
determine an exact homology of segments in Cyclostomes and Selachii, 
assumes the exact homology of the M. rectus posterior oculi 
of these two groups. This muscle is derived in Selachii, as has been 
shown by vAN WIJHE (’82) and others, from van WiJHE’s 3rd somite, 
and is without doubt the myotome of that somite. HATSCcHEKR infers, 
chiefly from the evidence that in Ammocoetes of late stages of 
development the M. rectus posterior “setzt sich an den mittleren 
Teil des ersten Myomers — dessen oberflächliche Schicht sich aber 
weit über ihn vorschiebt — und zwar an dessen vorderes Myoseptum, 
und zieht als gerade Fortsetzung jenes Muskelteiles nach vorne zum 
Auge” that these are two successive myomeres and the exact homologues 
of van Wısue’s 3rd and 4th somites in Selachii. However, these 
relations of the M. rectus posterior and the first myotome of 
the M. lateralis dorsalis are not primary, as Harscuex himself 
would have seen had he studied their development. Moreover, the 
relations are not so intimate as HATScHEK’s statement would give us 
reason to infer. I find that in an embryo of P. Planeri of 5 cm, the 
M. rectus posterior appears as a differentiated portion of the 
connective tissue capsule surrounding the eye. The cells of the muscle 
stain in precisely the same way as those of the surrounding tissue, 
but their elongated form and grouping proclaim them as the Anlage 
of a muscle. Moreover, the relations of this muscle Anlage to the eye 
appear the same as those of the M. rectus posterior in Squalus 
acanthias !). It is not, however, directly connected with the first myotome 
of the M. lateralis dorsalis. Numerous deeply staining strands 
of connective tissue similar in appearance to the ligamenta inter- 
muscularia which are so marked in Petromyzon embryos extend 
from the connective tissue capsule of the eye to the inner side of the 
first myotome — chiefly to the dorsal strand of this myotome (M. late- 
ralis capitis anterior superior of Kuprrer). These strands 
of connective tissue cannot be regarded as forming a myoseptum, since 
they extend in the same direction as the long axes of the first myotome 
and of the M. rectus posterior. On the basis of this evidence 
alone I am unable to accept HarscHer’s conclusion that the M. 
rectus posterior and the first myotome of the lateral musculature 
represent two successive myotomes. Moreover, the study of the early 
1) I have not, however, been able to determine its innervation. 
