. The Development of the Cartilaginous Skull ete. in Necturus. 439 
and ventral parts of the mesothelia of the remaining arches. The 
dotted lines at the right of the figure indicate the union of the ventral 
part of the mesothelium of the second vagus arch with the dorsal 
part of the mesothelium of the first vagus arch. The union takes 
place in sections lateral to the section represented. 
In each arch the mesothelial tissue lies in the long axis of a 
surrounding tube of mesectoderm. The mesectoderm represented in 
fig. 2, pl. XVI, extends ventrally to the wall of the pericardium from 
which the mesothelial cords pass dorsalwards through the centre of 
each mesectodermic arch, sharply distinguished from this tissue by 
yolk differentiation. As the embryo grows the are described by each 
mesothelial cord becomes greater, and the tissue pushes its way from 
the centre of the arch to the postero-lateral margin, where, on the 
posterior arches, the buds of the external gills appear. This stage 
of development is represented in the series, figs. 1 ~—17, pl. XVI. The 
vertical part of the mesothelial cords is seen in section in figs. 1 a—1 e, 
while in figures 1¢d—17, the union of each successive cord with the 
wall of the pericardium is represented. Fig. 4, pl. XVI, shows in 
transverse section, the two-walled mesothelial plate which unites the 
mesothelial tissue of the hyoid and mandibular arches, seen in 
horizontal section in figs. 1% and 17. The anterior part of the 
mesothelial tissue of the mandibular arch is continuous. from side to 
side in sections that pass below the plane of the section shown in 
fig. 12. From this tissue, with little change of relative position the 
mandibular part of the m. mylo-hyoideus arises, which is inner- 
vated by the trigeminal nerve. The hyoid part of the mesothelial 
plate gives rise to the posterior part of the muscle mylo-hyoideus, 
which is innervated by the facialis. 
In the embryo of 12!/, mm, represented in the sections shown 
in figs. 5a—5 e, pl. XVII, the mesothelial tissue of the younger embryo, 
with very little change of position has been converted into muscular 
tissue. The muscles of the mandibular and hyoid arches are con- 
tinuous with the muscular tissue of the mylo-hyoideus, as in the 
younger embryo the mesothelial tissue of these arches united in the 
ventral mesothelial plate. In the section shown in fig. 5 2, the large 
mandibular muscle is seen to send a band of muscle fibres to the 
wall of the branchial chamber. The embryonic muscles appear to 
come into action before the branchial bars are sufficiently firm to 
act as levers, and primarily extend from the dense mesectodermal 
tissue of the arch in which they“lie to an adjacent branchial wall. 
