AUTHOR’S ABSTRACT OF THIS PAPER ISSUED BY 
THE BIBLIOGRAPHIC SERVICE, SEPTEMBER 28 
VERTEBRATE CEPHALOGENESIS 
V. ORIGIN OF JAW APPARATUS AND TRIGEMINUS COMPLEX— 
AMPHIOXUS, AMMOCOETES, BDELLOSTOMA, CALLORHYNCHUS 
HOWARD AYERS 
THIRTY-SIX FIGURES 
The current theory of the origin of the vertebrate jaws assumes 
their derivation and descent from a pair of gills, with all the 
structural implications involved in that assumption. The food 
canal antedated gills, mere appendages of the wall of the food 
canal. The anterior and posterior openings of the canal un- 
doubtedly antedated not respiration, but the complicated bran- 
chial respiration of vertebrates. Branchial respiration in these 
forms is carried on by means of the richly vascular walls of holes 
through the gut and body walls. 
The phylogeny of these structures is insufficiently indicated 
by their ontogeny in the lower vertebrates, and comparative 
anatomy has not as yet revealed the story of their descent. 
In any event, the armature of the anterior opening of the food 
canal antedated the highly specialized gills of vertebrates as 
we yet know them, and all the evidence we have on this subject 
indicates that the jaws are in no way genetically related to 
branchial structures of any kind. 
»The testimony of Amphioxus, Ammocoetes, Bdellostoma, 
and Callorhynchus, bearing on this subject, is here presented 
for the purpose of solving a few of the host of questions which 
are involved in the phylogeny of the vertebrate jaws. Two 
conclusions that the testimony fully sustains are: 1) That the 
jaw apparatus is in no way related to gills, but is genetically 
related in the forms mentioned in the order named, and since 
Callorhynchus is an accepted Gnathostome, it follows that the 
Amphioxine jaw apparatus is the earliest known condition of 
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