The Development of the Cartilaginous Skull ete. in Necturus. 447 
fig. 21, the first 6 divisions of the postotic mesoderm are seen. The 
first division lies partly below the auditory vesicle, and partly below 
the glossopharyngeal ganglion, as in fig. 20. The second division 
of the postotic mesoderm half surrounds the vagus ganglion, while 
the third and following divisions are of the usual shape and size. 
In fig. 22 one sees the first three postotic somites of fig. 21 in a 
more median plane of section, where the dorsal limit of the posterior 
part of the first and second postotic somites rises to the plane 
bounding the dorsal wall of the third and following somites. 
Figs. 23 and 24 show sagittal sections from a still older embryo 
in which the auditory vesicle is nearly separate from the external 
ectoderm. Fig. 23 shows the more median section. In fig. 24 the 
first postotic somite, and also the ventral part of the second somite 
are represented by scattered cells of the mesenchyma. The position 
of the two somites, however, is easily determined by their relation 
to the glossopharyngeal and vagus ganglia. The cells of the dorsal 
part of the second somite are beginning to elongate into muscle fibres, 
and in the more median section shown in fig. 23, it is seen that 
these dorsal muscle fibres are continuous with the dorsal 
part of the third postotic somite. The line separating the two 
somites is still apparent, but ultimately no segmental division in the 
band of muscle fibres formed from the dorsal part of the second 
and third somites is found. Muscle fibres also arise in the group 
of cells seen in fig. 23 in the postero-ventral part of the second 
somite. These fibres extend anteriorly and inwards to the lateral 
surface of the chorda. In horizontal and transverse sections the relations 
here described are deceptive, and in my second study on Necturus 
(96, page 506), I mistook the partial fusion of two myotomes for a 
partial division. I found that the branchial clefts arise intersegmentally, 
the second or hyobranchial cleft lying beneath the auditory invagi- 
nation, between the fourth and fifth somites (v. WIJHE); the third 
cleft arises in the plane separating the fifth and sixth, first and 
second postotic, somites, and the fourth cleft between the second 
and third postotic somites. 
Owing to the fusion of the dorsal parts of the second and third 
postotic somites, it appeared that when the fifth cleft arose, it also 
lay between the second and third somites, while the fourth cleft 
appeared to have migrated forwards and to lie beneath a septum 
which divided, as I thought secondarily, the ventral part of what 
seemed to be the second somite. It now appears, however, that 
