The Development of the Cartilaginous Skull ete. in Neeturus. 411 
of neighboring cells has here ceased. More frequently, however, the 
prochondral tissue passes gradually from the compact structure of 
the bar into the less compact surrounding tissue, indicating that in 
these regions growth by aggregation of more scattered mesectoderm 
cells continues. 
Figs. 6 a—6 e, pl. XVII, from horizontal sections through the bran- 
chial region of an embryo 15 mm in length, are given for comparison 
with figs. 5@—5e. The plane of section inclines upwards from right 
to left, but because of the slight obliquity seemed to offer better 
opportunity for comparison, since the changes in the relative position 
of the tissues due to lifting the head into the plane of the long 
axis of the embryo have been considerable, and sections through an 
embryo of 12!/, mm do not exactly correspond with those through 
the embryo of 15 mm. 
Fig. 6a@ shows a section which passes through the optic stalk 
on the right, and below the median plane of the eye on the left. 
The group of prochondral cells posterior to the optic stalk, and the 
corresponding opposite group, are from the anterior horns of a primi- 
tive plate of tissue which underlies the brain. With these horns the 
anterior parts of the trabecular bars fuse, but the cells that compose 
the horns are not mesectodermie. A small group of cells, fr’, at 
either side of the bypophysis in fig. 5a are from these anterior horns 
of the basi-cranial plate at an earlier stage of development, when 
the cells that are to form the anterior part of the trabeculae are 
still indistinguishable in the mesectoderm which bounds the mouth 
anteriorly. 
Since the plane of fig. 5@ passes above the mandibular muscle, 
the relation of this muscle to the prochondral cells of the mandibular 
bar in fig. 6a, and 65 may be compared with the relation of the 
muscle to the closely grouped mesectoderm cells of the mandibular 
region in fig. 5%, or the position of the mandibular muscle and the 
procartilage at the left in fig. 6a might better be compared with the 
corresponding tissues in fig. 1c. In regard to the mesectoderm cells 
that lie external to the mandibular mesothelium in fig. 1c, I am 
unable to say whether they become absorbed in the procartilage which 
arises posterior to the mandibular muscle, or continue their migration 
downwards. 
The hyoid muscle and procartilage in fig. 6@ may be compared 
with the corresponding tissues in fig. 5a. The muscular tissue still 
occupies the posterior part of the arch, but on the right in fig. 6« 
