396 Julia B. Platt 
posterior to the optie vesiele is from the dorsal part of the mesothelial 
tissue of the mandibular arch. The position of the neighboring 
mesectoderm cells is seen to be similar to that of the mesectoderm 
cells of the hyoid arch in fig. 15, or of the glossopharyngeal (third 
visceral) arch in fig, 1a, since the respective sections cut these arches 
at nearly the same relative level. There are, however, a greater 
number of mesectoderm cells at the base of the mandibular arch 
than at the bases of the other arches, and unlike the mesothelium 
of the hyoid arch, the mesothelium of the mandibular arch is 
completely separated, at the level of the section, from the superficial 
ectoderm by intervening mesectoderm. 
One notices chiefly in the hyoid arch the relatively large amount 
of mesothelial tissue, and the absence of large blood vessels. Other- 
wise the arrangement of the median tissues of the hyoid arch in 
fig. 1c, is not unlike that in the third and fourth branchial arches of 
fig. 1 5. 
In the third branchial arch of fig. 1c, one finds the number of 
mesectoderm cells in the inner part of the arch greatly increased. 
This is partly, but not entirely, due to the obliquity of the plane 
of section in reference to the axis of the arch. There is, however, 
an actual accumulation of mesectoderm cells in the ventral part of 
the arch. 
A similar increase in the number of mesectoderm cells is found 
in the fourth arch, moreover the mesectoderm of this arch is now, 
in fig. 1c, united with that of the fifth arch near the wall of the 
pericardium. A small knob of cells on the antero-lateral wall of 
the pericardium is from the mesothelium of the fifth arch as it 
unites with the pericardium. The mesothelial tissue on the lateral 
wall of the pericardium, posterior to the knob of cells just mentioned, 
forms the anterior extremity of the ventral longitudinal muscles of 
the trunk. 
Fig. 19 passes through the middle of the eye and sections the 
floor of the branchial chamber. The small lumen in the wall of 
endoderm is a pocket from the alimentary canal which extends 
ventrally towards the mouth invagination. The entrance to the mouth 
is still entirely closed by endoderm, as seen from the following 
sections (figs. 14 and 12). The anterior of the paired projections 
in the lateral boundary of the endoderm (fig. 19) indicate the position 
of the mouth, which lies ventral to them. The posterior projections 
are from the ventral walls of the hyomandibular clefts. On the left 
