398 Julia B. Platt 
third arch. At either side of the head, the section (fig. 1g) cuts a 
stem of the external gills, which has budded from the third branchial 
arch. Fig. 1c passes through the base of a similar bud from the 
fourth arch. 
The tissue included in the line g’—g’, is drawn with higher 
magnification in fig. 1g’, and colored as it appears on the slide. 
The figure shows the ventral part of the right glossopharyngeal arch. 
Bounding the section above and at the right, one finds the endodermie 
wall of the second branchial cleft, which, at the level of the section, 
is separate from the floor of the branchial chamber. In the lower, 
right-hand corner of the figure, a mass of endodermic tissue, dis- 
tinguished from the ectoderm by the number of yolk granules, forms 
part of the ventral wall of the third branchial cleft. The tissue with 
yolk granules in the centre and at the left of the figure is meso- 
derm. The mesectoderm is nearly or quite free from yolk. 
Comparing fig. 1g’ with fig. 1 4’, one notices that the mesothelial 
tissue, which in fig. 15’ occupies an excentrie position at the outer 
and posterior part of the third branchial arch, retains its primitive 
axial position in fig. 1g', where the mesothelium of the arch, in 
uniting with the wall of the pericardium, changes from the vertical 
to the horizontal plane. Fig. 1g’ also shows that the mesothelial 
tissue of the third arch unites with the wall of a vascular space 
anterior to the pericardium as well as with the wall of the pericar- 
dium itself, forming together an apparently homogeneous mesoderm. 
This vascular space, the ventral aorta, or heart, is bounded, however, 
by cells at least in part of endodermie origin. The mesectoderm 
cells in fig. 1g’ are seen to be much more numerous than those of 
fig. 16’. The tissue they compose, moreover, is continuous in fig. 19’, 
while in fig. 1’ it is somewhat broken by cells of the mesenchyma 
connected with the vascular system. 
Fig. 14 passes through the lower part of the optic Vesiohll and 
shows the union of the mesothelial tissue of the hyoid arch with. 
the wall of the pericardium. The endoderm of the section is a solid 
mass of tissue in which the mouth cleft has not yet appeared. On 
comparing this section with those immediately preceding, it is seen 
that the endoderm which adjoins the anterior pericardial wall in 
fig. 1e, lies at some distance from the wall of the pericardium in 
figs. 1 f and 1g, but now again in fig. 1 comes into contact with 
the pericardial wall. Hence the bridge of mesectoderm which, in 
the region of the second and third branchial arches underlies the 
