The Development of the Cartilaginous Skull ete. in Necturus. 441 
where it is also seen that the dorsal part of the hyoid mesothelium, 
that part which connects the mesothelium of the hyoid arch with 
that of the glossopharyngeal arch, forms a distinct mass of tissue, 
immediately adjoining the more ventral hyoid mesothelium. ‘This 
dorsal tissue, which lies below the primary centre of chondrification 
of the auditory capsule, gives rise to the digastric muscle. The 
muscle retains its primary attachment to the postero-ventral wall of 
the auditory capsule, but also becomes attached to the dorsal part 
of the cartilaginous bar of the glossopharyngeal arch, and is ulti- 
mately connected with the posterior extremity of the mandibular bar 
by a long tendon. 
The more ventral of the two divisions of the hyoid mesothelium, 
seen in fig. 26, gives rise to the muscle cerato-hyoideus externus, 
which is widely inserted on the hyoid cartilage, and also becomes 
attached to the dorsal part of the cartilaginous bar of the glosso- 
pharyngeal arch, near the point of attachment of the digastric 
muscle. Both of these muscles are innervated by the hyoid nerve. 
The third and most ventral of the hyoid group of muscles is the 
mylo-hyoideus, the origin of which has been described. In fig. 7 d, 
the three hyoid muscles are seen in transverse section (dig. che. myh.). 
In fig. 6 4, the cerato-hyoideus ext. and the mylo-hyoideus are 
seen in horizontal section. 
From the remainder of the dorsal mesothelium seen in fig. 26, 
the levatores arcuum arise. That division of this mesothelium 
which is continuous with the hyoid mesothelium gives rise to the 
levator arcus I, which, with little change of position, becomes 
connected proximally with the bones investing the posterior auditory 
region, and distally with the dorsal part of the cartilage of the 
glossopharyngeal arch. The proxinal part of the muscle, however, 
grows inwards, and ultimately passes beyond the external boundary 
of the myotomes, which lie in planes median to that shown in fig. 26. 
The glossopharyngeal nerve innervates this dorsal muscle of the 
glossopharyngeal arch. From the posterior part of the dorsal meso- 
thelium, seen in fig. 26, the two levatores of the vagus arches 
arise, innervated each by a branch of the vagus nerve. 
Mivarr (69, page 456) calls attention to the fact that the levator 
muscle of the first arch, in the adult Necturus, is remarkably dis- 
tinct from the remaining levatores arcuum. Fig. 26 shows that 
even the mesothelium from which the muscle arises is less closely 
connected with the mesothelium of the vagus arches than with that 
