404 HOWARD AYERS 
in a subsequent contribution. But even though it could not 
be shown that they are homologous, how much will that uncer- 
tainty detract from the body of fact which shows that the more 
or less longitudinal muscles of the jaw of Amphioxus arise out 
of the transversus, that the transversus is innervated by the 
nerves of the jaw apparatus, that in Ammocoetes these muscles, 
although attached to the skull on both sides and continuous 
across the midventral line, bear the same relations to the jaw bars, 
are innervated in this territory by the nerves of the Jaw apparatus 
(trigeminus), and further only differ from those of Amphioxus in 
having developed two layers with skeletal tissue between them; 
that in Bdellostoma they are split up into segments of circles 
and have attachments to numerous cartilages, although they 
still maintain the same fundamental relation to the jaw-bars 
and continue to be innervated by the nerves of the Jaw apparatus? 
In the face of such facts, what is a denial of their homology worth? 
Take the old-time puzzle—the location of the nerves of Amphi- 
oxus outside the myotomes and their submyotomic position in 
all the other vertebrate forms. Even if we cannot here solve 
this problem, does the fact that, at some time and somehow, the 
muscle seems to have disappeared from under the proximal part 
of the nerves of the jaw apparatus invalidate the evidence that 
nerves supplying homologous structures in Amphioxus and 
Bdellostoma are not themselves homologous? In other words, 
does a non-essential fact destroy the value of an essential fact? 
Other problems like these might be mentioned, but none seem 
to me to constitute valid objections to the conclusions drawn from 
the facts that have been presented, viz.: That the Amphioxine 
jaw apparatus is the parent structure out of which has been 
evolved the mandibular mechanism of the Gnathostome’ ver- 
tebrates and its hyoidean companion mechanism. That to 
branchial structures the jaw has no genetic relationship, while 
to the trigeminus complex it bears the relation of end-organ. 
The mystery of the labial cartilages dissolves when we recog- 
nize their origin, follow their unfolding, obsolescence, and final 
disappearance. . 
Winding Way and Valley Road, Cincinnati, Ohio, 
April 27, 1921 
