390 Julia B. Platt 
Vertebrate, acquired before the mesoderm had assumed a share in 
the functions originally belonging to the ectoderm. It may be 
thought that the partieipation of mesectoderm in the formation of 
dentine tells against this view. However, the tooth, or placoid scale, 
stands on the border line where the external ectoderm first calls 
the lower tissue to its aid in forming a defensive skeleton, in which 
enamel, a product of the ectoderm, was first and originally the only. 
hard substance (WIEDERSHEIM, 793, page 45). 
This introduetion does not claim the merit of an historical or 
critical review, since many works of importance, which might have 
been mentioned, have been omitted, while those of less accuracy 
have been coordinated with those of greater. My purpose has been 
to show the position which this study holds in relation to morpho- 
logical questions still undecided, thus possibly justifying the detailed 
description which follows. 
Part I 
Procartilage. 
As the present study begins with a stage of development im- 
mediately following that with which my first study (94) on the 
ontogenetic differentiations of the ectoderm in Necturus 
closed, I briefly review those facts recorded in the earlier publica- 
tion which serve to explain the condition of the middle tissues in 
the head of the embryo about to be described. 
a) In Review. 
Prior to the closure of the neural folds in Necturus, one finds 
that the wings of axial mesoderm at the side of the chorda end 
anteriorly in the same transverse plane as the chorda itself. Anterior 
to this plane, the wall-of the archenteron comes into immediate 
contact with the ectoderm, touching the floor of the neural plate 
medianly and forming laterally, by contact with the ectoderm at the’ 
side of the neural plate, the Anlagen of the hyomandibular clefts, 
which consequently do not break through the mesoderm, but are 
secondarily surrounded by mesoderm. A similar mode of formation 
of the anterior branchial clefts in Petromyzon is described by v. 
KUPFFER (94). 
Anterior to the extremity of the chorda, the dorsal wall of the 
archenteron gives rise to mesoderm primarily quite separate from 
