394 Julia B. Platt 
facial ganglion, seen at the left in the more dorsal section (fig. 4 a) 
has disappeared in fusing with the sensory epithelium of the supra- 
orbital line, and the posterior part of the ganglion lies in this section 
(fig. 15) near the mesothelial tissue of the hyoid arch. The meso- 
thelial tissue of the hyoid arch, as well as that of the third, fourth, 
and fifth branchial arches, lies in the external and posterior part 
of the arch where it forms a cord of tissue extending vertically. 
The plane of fig 15 sections transversely these cords of mesothelial 
tissue. Aside from the mesothelial cells, the posterior branchial 
arches also contain many scattered mesoderm cells that take part in 
the formation of the vascular system. 
The mesectoderm in the region of the trigeminal and facial 
ganglia (fig. 16) lies chiefly near the external ectoderm, but several 
cells are also seen at the median side of the mesothelium of the 
hyoid arch. Above the plane of section, the external and median 
groups of mesectoderm cells are connected with one another, and in 
the embryo younger than that now described, the mesothelium of 
the arches is separated in its entire vertical extent from the external 
ectoderm by a layer of mesectoderm, and lies as a cord in the centre 
of a surrounding tube of mesectoderm. As development proceeds, 
however, the mesothelial tissue makes its way from the original, 
central, position towards the external and posterior surface of the 
arch, where it is seen in fig. 12. 
In the posterior branchial arches, the same shifting of the meso- 
thelium from its primary, central, position oceurs, and the relations 
of the median branchial tissues to one another are further compli- 
cated by the migration of cells of the mesenchyma through the 
mesectoderm in consequence of the development of the vascular system, 
as the walls of the blood vessels are formed by cells of mesodermic 
(or endodermic) origin. 
To show the great difference in the aspect of the two median 
tissues, mesoderm and mesectoderm, parts of three ‚sections have 
been enlarged and colored as they appear when stained with 
Bronpi-Euruicu. The third branchial arch on the left of fig. 14, 
included in the line 6’—d’, was chosen for enlargement in fig. 10’. 
On three sides of the figure, above, below,. and at the right, are 
seen yolk-laden cells from the wall of the alimentary canal. The 
bounding tissue at the left is ectoderm. In the lower, left corner 
of the figure, an irregular group of yolk-laden mesoderm cells repre- 
sents the mesothelial cord in transverse section, while scattered cells 
