438 Julia B. Platt 
Mıvarr (69), closely following FiscuEr (64), has given an account 
of the adult muscles in Menobranchus (Necturus) which would 
also serve as descriptive of the muscles in Necturus of 46 mm, 
and I consequently refer to his paper for the final relations and 
insertions of the muscles, having nothing further to add to the accu- 
racy of his description. 
The branchial muscles. 
In the embryo of 11mm, the mesodermic tissues of the head 
may be divided into three groups, — mesenchymal, mesothelial, and 
somitie tissues, and it is the mesothelial tissue which gives rise to 
the branchial muscles. This tissue forms the wall of the pericardium 
(coelom) and thence extends dorso-laterally or antero-dorsally into 
each branchial arch. The mesothelium of the mandibular arch is 
connected with the wall of the pericardium through the mediation 
of the ventral part of the hyoid mesothelium. In other words, the 
mandibular and hyoid mesothelia unite with one another ventrally, 
while the hyoid mesothelium unites with the anterior wall of the 
pericardium. The pericardial cavity extends into the ventral part 
of the mesothelium of the hyoid and mandibular arches alone, while 
in the dorsal part of these arches and in the posterior branchial 
arches, the body cavity, theoretically present, is completely reduced 
by the approximation of the cells composing the mesothelial walls. 
At its dorsal extremity the mesothelium of the mandibular arch 
is separate from the mesothelium of the following arches, and passes 
directly into mesenchyma. The mesothelium of the hyoid arch extends 
dorsally to the ventral wall of the membranous ear, as shown in 
fig. 11. Below the posterior wall of the ear, the mesothelium of the 
hyoid arch is directly continuous with the dorsal part of the meso- 
thelial tissue of the glossopharyngeal arch, as seen from the sagittal 
section represented in fig. 26, pl. XVIII, where it is also seen that 
at some little distance above the branchial clefts, the dorsal extremity 
of the mesothelium of the glossopharyngeal arch is connected in turn 
by a somewhat broken band of mesothelial tissue with the dorsal 
part of the united mesothelia of the vagus arches. Thus the meso- 
thelium of the mandibular arch is dorsally independent, while the 
mesothelial tissue of the following arches is continuous above the 
gill clefts. 
These relations are shown in fig. 26. The plane of section passes 
vertically through the hyoid mesothelium, but sections only the dorsal 
