The Development of the Cartilaginous Skull ete. in Necturus. 391 
the wings of axial mesoderm at the side of the chorda!. The dorsal 
wall of the archenteron is thus the source of that mesoderm which 
represents in Necturus the walls of the mandibular, praemandibular, 
and »anterior« head-cavities in the Selachii. These head-cavities, 
however, are not distinguishable in the endodermie mesoderm of 
Necturus. 
In later development, the anterior mesoderm, thus proliferated 
from the dorsal wall of the archenteron, unites with the wings of 
axial mesoderm, both above and below the hyomandibular clefts, 
forming with the axial mesoderm an apparently homogeneous tissue. 
In my first study on the differentiations of the ectoderm (94, 
page 913), I assumed, on the authority of the majority, that the axial 
mesoderm, which arises at the margin of the blastopore, or in the 
line of the primitive streak, is morphologically a tissue .of purely 
endodermic origin, and in distinction from later ectodermic additions 
to the »middle layer«, I called this presumably endodermie mesoderm 
»mesendoderm«. Subsequent study leads me to doubt whether the 
axial mesoderm can properly be regarded, even theoretically, as a 
derivative of the endoderm alone, and in consequence, I return to 
the familiar appellation »mesoderm«, using the term, however, to 
designate a tissue composed of cells of uncertain origin and affinities. 
The neural crest in Necturus is interrupted between the 
trigeminal and facial Anlagen. Both parts of the crest fuse with 
thickenings of the ectoderm which occur in two longitudinal lines 
on each side of the head; in a dorso-lateral line which passes through 
the auditory epithelium, and in an epibranchial line, immediately 
above the dorsal margin of the gill clefts. Along both lines of ecto- 
dermic thickening cells are rapidly added to those of the neural crest 
constituting together a tissue, the mesectoderm, peculiarly distinguished 
in Necturus from the surrounding mesoderm by great reduction in 
the size of the yolk granules with which all of the embryonic tissues 
are primarily crowded. Similar ectodermic thickenings from which 
cells are added to those of the neural crest have been described by 
different authors in several classes of Vertebrates, and according to 
the detailed description of v. Kuprrer (90, ’91) the relations in 
Petromyzon chosely resemble those in Necturus. | 
1 This region of mesodermic proliferation is axial and does not extend 
laterally, relatively ventrally, to the region in which the mandibular arteries, 
seen in fig. 19, pl. XVIII, arise. 
