418 Julia B. Platt 
the basieranial plate in the intersegment between the second and 
third primitive myotomes, or third and fourth postotie somites. The 
first somite posterior to the ear, relatively below the ear, in the 
primary divisions of the mesoderm segmentally related to the glosso- 
pharyngeal nerve and to the third visceral arch, does not develop 
a proper myotome, but soon becomes scattered in mesenchyma. 
In the transverse planes of the myosepta posterior to the 
occipital arch, the mesenchyma at the side of the chorda dorsalis 
and spinal cord begins to form a more compact tissue in preparation 
for the neural arches soon to appear. In the embryo of 15 mm, there 
is as yet no cartilage. 
Fig. 9, pl. XVIII, shows the ventral aspect of a model of the wall 
of the branchial chamber in an embryo of 15 mm. From comparison 
of this figure with fig.3, one sees the changes in the shape and 
position of the branchial clefts that are correlated with the conversion 
of mesectoderm into procartilage. The first transverse fold, seen at 
the right in fig. 9, represents the oral fusion. Anterior to this fold 
lie the trabeculae and almost immediately posterior lie the mandibular 
bars. The second transverse fold, representing the hyomandibular 
cleft, does not now extend to the surface of the embryo as in the 
earlier stage of development shown in fig. 3. The median outgrowth 
from the floor of the branchial chamber, the thyroid Anlage, still 
extends backwards from the floor of the united hyomandibular pockets 
to the wall of the pericardium. Dorsal to the anterior extremity of the 
thyroid outgrowth, and beneath the hyomandibular fold, lies the centre 
of the hyoid bar, from which the »horns« extend on each side out- 
wards and backwards to the angle where the hyomandibular fold 
is lost in the wall of the branchial chamber. The »first basi- 
branchial« lies in the axial plane immediately beneath (dorsal to) 
the thyroid outgrowth, and extends backwards between the floor of 
the branchial chamber and the third endodermic fold, seen in fig. 9, 
under the median part of which the first basibranchial unites with 
the bar of the glossopharyngeal arch. 
The third transverse fold is formed by the hyobranchial clefts, 
which are not at first united, but extend directly outwards, as seen 
in fig. 3. Later, the tissues of the hyoid arch grow backwards 
forming a kind of operculum, and the direction of the hyobranchial 
clefts changes accordingly from the transverse plane to an oblique 
one. As the hyobranchial clefts change their direction, the external 
part of the clefts elongates ventrally, until the two clefts meet and 
