The Development of the Cartilaginous Skull ete. in Necturus. 399 
floor of the branchial cavity, is bounded ventrally as well as dorsally 
by endoderm. These relations are more easily explained from the 
models soon to be described. 
The mesothelial tissue of the mandibular arch, in fig. 1%, has 
passed the outward bend in the lateral wall of the endoderm, opposite 
which it stood in fig. 19, and now lies posterior to the transverse 
plane of the mouth. In the mesothelium of the hyoid arch a small 
central cavity is seen. Although the mesothelial tissues of the several 
arches undoubtedly represent the walls of the branchial part of the 
body cavity, in Necturus the mesothelium of the branchial arches 
usually forms a compact cord of cells in which the central cavity 
is obliterated, save in the ventral part of the hyoid and mandibular 
arches. 
In fig. 1%, as compared with fig. 19, the number of mesecto- 
derm cells has further increased, and their nuclei are more closely 
crowded together in the posterior part of the section, while in the 
anterior part of the section, scattered mesectoderm cells now replace 
the mesenchyma of more dorsal sections, save at the surface of the 
brain, which is bounded by a vascular network of mesenchymal cells 
i. e. scattered cells of mesodermic origin. 
The section shown in fig. 17 passes through the optic stalk at 
the right, and through the dorsal part of the nasal epithelium at the 
left. The endoderm extends to the lateral surface of the section, 
where the dorsal (ultimately posterior) margin of the mouth lies. The 
posterior prolongation of the endoderm seen in fig. 14 has receded 
from the pericardial wall, and the short intervening space is occupied 
by cells of the vascular mesenchyma. The mesothelial tissue of the 
mandibular arch has fused with that of the hyoid, thus indirectly 
establishing its connection with the pericardial wall. A mandibular 
head-cavity is found on both sides of the embryo. The mesectoderm 
in the anterior part of the section, shown in fig. 12, includes more 
cellular elements than in fig. 14, but is constantly separated from 
the brain by an irregular line of mesenchyma. The scattered mesecto- 
derm cells of the anterior part of the head gradually form a more 
compact tissue as they approach the anterior (dorsal) surface of the 
mouth, which now separates them from the mesectoderm of the 
mandibular arch. 
The tissues included within the curved line 7’—7 of fig. 1%, 
are colored and enlarged in fig. 12’. At the left this figure is bounded 
by ectoderm, in which the yolk granules are not yet entirely absor- 
