The Development of the Cartilaginous Skull etc. in Necturus. 393 
relation of the two middle tissues (mesectoderm and mesoderm) 
immediately before the differentiation which may be called the pro- 
cartilage stage. 
Fig. 1a shows the most dorsal section of the series. At the 
left, the section passes through the floor of the auditory vesicle, and 
on the right slightly above the floor. The section cuts the anterior 
end of the notochord and the dorsal wall of the alimentary canal. 
The dorsal part of the third and fourth branchial clefts appears in 
section, but the plane of section lies above the dorsal margins of 
the first, second, and fifth clefts!. The large ganglia of the glosso- 
pharyngeus and vagus nerves lie above the plane of section, and 
are now separate from the connective tissue component of the primi- 
tive mesectoderm, through the dorsal part of which the section passes, 
as this tissue begins to turn inwards at the dorsal margin of the 
branchial clefts. 
The mesoderm seen in fig. 1a is chiefly composed of mesenchyma, 
but in each of the branchial arches a closer grouping of the cells 
gives rise to a mass of tissue (mes) more or less epithelial in 
character, which is the precursor of muscular differentiation. In 
each arch the mesectoderm nearly surrounds this mesothelial tissue. 
The section of fig. 14 passes through the cavity of the branchial 
chamber. The anterior (dorsal) wall has fallen away from the 
mesoderm, as easily happens in preparations. The plane of section 
passes through the second and fifth branchial clefts in addition to 
those of the more dorsal section. The root of the trigeminal ganglion 
lies between: the sections shown in figs. 1a and 1d, and the ganglion 
now (fig. 14) appears in two divisions. The anterior part of the 
1 Although the hyomandibular cleft does not break through in Necturus, 
I begin my enumeration with this cleft, and also count the mandibular arch 
as the first branchial arch. The method of designating the whole series of 
arches >visceral«, and of beginning the enumeration of the »branchial< arches 
with the third visceral, meets with difficulties in comparative embryology since 
in Reptilia, Aves, and Mammalia, where embryonic arches also exist, the 
' third ‘visceral arch is not truly branchial, but, like the hyoid and mandibular 
arches, subserves merely secondary functions, while the third visceral arch is 
not the first branchial, i. e. gill-bearing, in the Selachii or Ganoidei (GE- 
GENBAUR '78). I therefore prefer to follow BALFOUR (85) in using >branchial« 
as coextensive with »visceral<, and shall designate the individual arches that 
lie posterior to the mandibular and hyoid, as the glossopharyngeal arch, the 
first vagus, and the second vagus, in correspondence with their primitive nerve 
supply, which is constant throughout the Vertebrata. 
