340 HOWARD AYERS 
the gnathostome jaw. 2) That the jaw apparatus is the end- 
organ of the trigeminus. These two conclusions are presented 
before the evidence, in order that the reader may better weigh 
the evidence given. 
In the past the study of the jaw began with the examination 
of the human jaw and worked down through the long series of 
vertebrate forms, seeking evidence to homologize and explain 
the origin and transformation of these parts in the vertebrate 
stock. Embryological evidence was abundantly used to check 
the results and to clarify the relationships. A theory of the 
origin of the jaw was thus worked out, and it became generally 
accepted among morphologists that the jaw was derived from 
one of the anterior gills, presumably the first pair of the ancestral 
vertebrate. However, indications of presumptive premandib- ~ 
ular structures were found which cast doubt on the accuracy 
of this conclusion as regards the particular pair of gills involved. 
The Myxinoids could not be brought into this arrangement 
because they were held to have no jaws. They were set aside 
in a special group—Cyclostomes—and contrasted with the 
Gnathostomes. Amphioxus was considered to have nothing 
even suggesting Jaws. 
The jaw apparatus plays such an important réle in cephali- 
zation of the vertebrate body that I have for quite some time 
given special attention to the subject, particularly to what 
Amphioxus, Ammocoetes, and Bdellostoma have to tell, and 
I think they give a clear, straight story of how the jaw came to be. 
The material used in this study consisted: 1) of adult and young 
stages of Amphioxus from the Naples Station treated with dif- 
ferent reagents. 2) Ammocoetes and Petromyzon from Cayuga 
Lake, consisting of a series of stages covering the whole Ammo- 
coetes period, stages illustrative of the metamorphosis and heads 
of the adult Petromyzon. It is a pleasure to acknowledge my 
indebtedness to Prof. Simon H. Gage, of Cornell University, 
for this adequate material, which was of great value in my in- 
vestigation. I am particularly grateful to Professor Gage for 
his generosity in supplying me with Ammocoetes undergoing 
metamorphic changes. 3) Bdellostoma from the Bay of Mon- 
