526 ALLIS. [Vou. XII. 
structures. It is also evident that this want of homology is 
found entirely in those muscles that are innervated by the 
oculomotorius and abducens, that is, in those muscles that are 
said to arise in all the Gnathostomata from van Wijhe’s first 
and third somites. As the several muscles that operate the 
eyeball have been developed from the muscle masses in these 
two somites, different arrangements have arisen, which, if it be 
assumed that reversions have not taken place, must indicate 
totally distinct and different lines of descent. The several 
arrangements characteristic of these lines may have arisen 
independently and simultaneously from the undifferentiated 
condition, or some one or two primary arrangements may have 
first so arisen, and from them, by later development, the exist- 
ing arrangements. This latter method of development seeming 
the more probable, I have attempted to construct a prototype 
from which existing arrangements might have been derived. 
The superior rectus in ganoids, teleosts, and Anura has 
unquestionably been developed from a muscle mass which has, 
in Elasmobranchii, Dipnoi, and Urodela, given origin to two 
muscles, the rectus superior and rectus internus. As the 
rectus internus in Holocephala arises near the front end of the 
orbit, at some distance from the place of origin of the rectus 
superior, it may be safely assumed that it represents a condition 
more primitive than that found in the Plagiostomata, where the 
two muscles arise together at the hind end of the orbit; for a 
muscle once having arisen with the other recti at the hind end 
of the orbit, would not, in all probability, have changed that 
place of origin for the apparently less advantageous one near 
the front end of it. It may, therefore, be assumed that the 
rectus superior of ganoids was formed by the fusion of the two 
muscles of elasmobranchs, rather than that it, by splitting, 
gave origin to the latter. The elasmobranch type would, 
therefore, be more primitive than the ganoid type, and, as a 
consequence, the internal and inferior recti of the latter would 
be formed by the splitting of the inferior rectus of the former, 
instead of by their fusion giving origin to it. We therefore 
conclude that in the prototype of all vertebrates, the Cyclosto- 
mata excepted, the superior branch of the oculomotorius inner- 
