420 Julia B. Platt 
by the chorda into two halves. The width and depth of the plate 
is less than in fig. 74, and its outline is less regular. Below the 
floor of the branchial chamber, the section passes through the first 
basibranchial in the axial plane, and through the hyoid bar at either 
side. The lateral part of the hyomandibular pocket lies external to — 
this bar of procartilage (see fig. 9), while internal to the bar are 
seen the dorsal extremities of the hyobranchial clefts. On the lateral 
wall of the hyomandibular cleft lies the posterior part of the mandibular 
muscle. Below the basibranchial bar, the thyroid outgrowth is seen 
in transverse section, while the wide mylohyoid muscle underlies all. 
A group of cells seen in fig. 7c below the auditory vesicles, 
gives rise to the first cartilage of the auditory capsule. I have not 
included the tissue which these cells compose in the model of the 
procartilage, although it somewhat resembles that of the basicranial 
plate. Figs. 11 and 12, pl. XVIII, are from transverse sections through 
the auditory vesicle and the surrounding tissues, before the constric- 
tions that ultimately divide the visicle have appeared, and before 
the yolk granules have disappeared from the mesoderm. The section 
seen in fig. 11 is from an embryo about 10 mm in length. The 
auditory vesicle is here bounded on the left by the facial (auditory) 
ganglion, and below by closely grouped mesoderm cells that lie at 
the head of the mesothelial tissue of the hyoid arch. 
Fig. 12 is from an embryo of 13 mm, and shows the opposite 
vesicle to that seen in fig. 11. As the magnification in figs. 11 and 12 
is the same, it is evident that the auditory vesicle is much larger 
in the older embryo. The epithelium in the vicinity of the facial 
(auditory) ganglion has become deeper, while the ventral wall of 
the vesicle has become thiner. In the younger embryo (fig. 11) the 
closely grouped mesoderm cells below the ear form an apparently 
homogeneous tissue with the mesothelium of the hyoid arch, in 
which no muscle fibres have yet developed. In the older embryo 
(fig. 12), the tissue in question is differentiated into two parts. In 
the ventral part muscle fibres have appeared continuous with those 
of the hyoid arch, while the dorsal part of the tissue is separated 
off as a plate of procartilage underlying the ear. To this plate as 
a centre of growth, mesoderm cells soon add themselves, enveloping 
the lateral wall of the auditory vesicle with a single layer of 
cartilage-forming cells. In the embryo from which a section is 
shown in fig. 12, these lateral cells have not yet differentiated from 
the sourrounding mesenchyma. 
