414 Julia B. Platt 
and near the wall of the pericardium the section also passes through 
the left constrietores arcuum. 
Median to the nasal epithelia, fig. 6 d shows the procartilage of 
the trabeculae, here formed from the mesectoderm on the anterior 
wall of the mouth. A relatively small part, however, of the dense 
mesectoderm which bounds the mouth anteriorly takes part in the 
formation of the trabecular bars. The thyroid outgrowth from the 
floor of the branchial chamber, which is seen in fig. 5d, lies just 
dorsal to the plane of section figured in 6d, where a fragment of 
the outgrowth, however, is seen at the left of the muscle genio- 
hyoideus. The region of the mandibular and hyoid arches seen 
at the left in fig. 6d, corresponds closely to that seen in fig. 5d. 
The dense mesectoderm at either side of the mandibular muscle in 
the younger embryo has given rise to the corresponding procartilage of 
the older embryo. In the hyoid arch, the muscle mylo-hyoideus 
of fig. 6d, occupies the same relative position as in fig. 5d. The 
surrounding mesectoderm cells have become less numerous in the 
older embryo owing to the aggregation of these cells in the pro- 
cartilage of the byoid arch. At the right, in fig. 6 d, the mandibular 
muscle passes into the mylo-hyoideus. Although more mesectoderm 
cells immediately take part in the formation of the procartilage of 
the mandibular arch than in the formation of the trabeculae, the 
cartilaginous bars of this arch by no means exhaust the mesectoderm 
that bounds the mouth posteriorly. 
In fig. 6e, the relation of the cells in the trabecular region 
resembles that in fig. 6d. The mandibular mesectoderm, seen posterior 
to the mouth in fig. 5e, has given rise in the older embryo to the 
mandibular bars of procartilage seen in fig. 6e. The position of the 
mylo-hyoid muscle is similar in both sections. 
The three series presented in figs. 1a@—17, 5a—5e, and 
6a—6e cover those stages of development in which the sharply 
differentiated mesectoderm of the branchial arches in the embryo of 
11 mm is converted into procartilage in the embryo of 15 mm. 
Aside from the procartilage of the branchial arches, a plate of 
peculiar tissue underlies the brain of an embryo of 15 mm, which 
resembles the procartilage of the branchial arches in also giving 
rise to true cartilage. The entire plate, however, is not converted 
into cartilage, although is seems primarily composed of a homogen- 
eous tissue. This tissue differs in appearance from the procartilage 
of the branchial arches in that the cells of which it is composed 
