The Development of the Cartilaginous Skull ete. in Neeturus. 407 
increased are of extension in the older embryo. Median to the 
fourth branchial cleft, fig. 5 « shows the suprapericardial bodies 
The fifth branchial cleft lies dorsal to the plane of section as in 
eee Le: 
At the left fig. 5 @ shows the union of the base of the muscle 
band of the second vagus arch with the muscular tissue on the wall 
of the pericardium, corresponding to the knob of mesothelial tissue 
on the antero-lateral wall of the pericardium in fig. 1c. The plane 
of fig.5 « passes posteriorly through a median depression in the 
floor of the alimentary canal. The mesectoderm uniting ventrally 
the first and second vagus arches on each side of the body in fig. 1 e, 
is united in the median line with that of the opposite side in fig. 5 a, 
showing that the axial fusion of the mesectoderm, begun with the 
hyoid bridge in the embryo of 10 mm (fig. 2) and extending to the 
glossopharyngeal arch in the embryo of 11 mm, now extends to the 
first vagus arch. 
Fig. 5 6 shows a section through the middle of the eye, and 
may be compared with fig. 19. In place of the mandibular meso- 
thelium posterior to the eye in the younger embryo, we find now 
the large mandibular muscle from which a prolongation extends in- 
wards to the wall of the branchial cavity. The nature of the cells 
grouped closely just posterior to the muscle, and through which the 
attenuated mandibular artery passes may be determined by compa- 
rison with fig. 1 g, where the only cells in the region in question 
are distinctly mesectodermic. The floor of the hyomandibular cleft 
is seen in fig. 5 4, while external to this prolongation from the wall 
of the branchial cavity, the mesectoderm of the mandibular arch 
unites with that of the hyoid arch, as on the right in fig. 1 g. The 
hyoid mesothelium has developed into the hyoid muscle, which 
begins to separate into two distinct parts, an inner thicker part, the 
muscle cerato-hyoideus externus, and an outer thiner part, the 
muscle mylo-hyoideus. Median to the hyoid muscle, and in the 
angle formed by the bases of the hyomandibular and hyobranchial 
pockets, the mesectoderm cells are closely grouped. The planes of 
figs. 1 g and 5 d, in both the younger and older embryos, pass 
through the mesothelium, or muscular tissue, of the glossopharyngeal 
arch as it unites with the wall of the pericardium. In the older em- 
bryo, however, an outgrowth from the mesothelium of the glosso- 
pharyngeal arch extends forwards towards the wall of the hyo- 
branchial cleft. This is the beginning of the muscle cerato-hyoideus 
