406 Julia B. Platt 
impracticable because the separation between meseetoderm and mes- 
enchyma (i. e. wandering cells of mesodermie origin) is uncertain 
in the older embryo, while the differentiation of the mesothelium — 
into muscular tissue gives a new means of distinguishing this tissue 
from the mesectoderm. 
Comparing figs. 5a and 1c, one notices that the planes of section 
correspond only approximately. Both sections pass through the in- 
fundibulum and through the dorsal part of the eye, but while fig. 1 ¢ 
shows the mesothelium of the mandibular arch, the plane of fig. 5 a 
passes above the dorsal limit of this tissue. The region occupied 
in fig. 1¢ by the mandibular mesothelium and the surrounding mes- 
ectoderm, is occupied in fig. 5 @ by tissue distinguished from the 
remaining reticular tissue of the anterior part of the section by the 
closer grouping of the component cells, and also by the entire 
absence of yolk granules which still distinguishes the mesectoderm 
from the adjacent mesenchyma, although the line of demarcation is 
no longer as sharp as in the younger embryo. 
In the hyoid arch the mesothelial tissue, seen in fig. 1c, has 
given rise to the large hyoid muscle, anterior to which in fig. 5 a, 
lies the hyoid nerve. The migration of the mesothelial tissue towards 
the posterior wall of the arch, begun in the younger embryo, has 
continued, and in consequence the mesectoderm of the older embryo 
occupies only the anterior part of the arch. The walls of the pos- 
terior branchial arches have expanded laterally with the growth 
of the external gills, and the are described by the mesothelial tissue 
of the arches has increased in length correspondingly. Although all 
of the muscular tissue in the branchial arches arises from the prim- 
itive mesothelium, all of the mesothelial tissue does not give rise 
to muscle cells, and one finds that part of the space once occupied 
by the mesothelial cord is ultimately occupied by vascular tissue. 
In the glossopharyngeal arch, the meseetoderm cells in the inner 
part of the arch have increased in number in the older embryo 
fig. 5 a), while the more open tissue which occupies the external 
part of the arch appears to be formed by cells of mixed origin. 
In the first vagus arch, fig. 5 « shows the cord of mesothelial 
tissue as it bends inwards towards the pericardium. In sections 
through more dorsal planes, this muscle band is sectioned trans- 
versely and occupies the same relative position as the muscle band 
of the glossopharyngeal arch in fig. 5 a. The mesothelial tissue of 
the vagus arch, like that of the glossopharyngeal arch, has also. an 
