The Development of the Cartilaginous Skull ete. in Necturus. 401 
and left ridges of ectoderm, seen in fig. 1 7, just anterior to the 
pericardial wall, meet and sever the mandibular arches from the 
wall of the pericardium. The point at which the mesothelial tissue 
of one side of the head becomes continuous with that of the other 
side is carried farther forward as the plane of section becomes more 
ventral, until the mandibular mesothelium passes out of section, and 
the outer mesectoderm of the mandibular arch becomes continuous 
in the median plane. As seen from fig. 1, the antero-ventral part 
of the head continues in section for some time, but as this part of 
the head, which ultimately becomes the antero-dorsal, is not directly 
concerned with the formation of cartilage, it seemed unnecessary to 
continue the series of figures beyond the plane of fig. 1 7. 
Hoping to make the relations of the median tissues of the head 
clearer, I also give in figs. 2 and 3, drawings from models which 
were made from cross-sections through an embryo 10 mm long. 
Fig. 2 represents the mesectoderm, and fig. 3, the branchial endo- 
derm. I chose for reconstruction an embryo slightly younger than 
that from which sections 1 @—17 were made, in order to show more 
primitive relations in the hyoid and mandibular arches. 
Although fig. 2 shows the space relations of the mesectoderm, it 
fails to give an idea of the density of the tissue represented. Thus 
the anterior mesectoderm of the head forms a reticulum with rela- 
tively large interspaces, while the mesectoderm that bounds the 
epithelium of the mouth, and that which occupies the ventral part 
of the branchial arches, is a dense tissue composed of continuous 
protoplasm, one might almost say composed of nuclei, so close to 
one another do the nuclei lie. The mesectoderm of the dorsal 
branchial region is intermediate in density, but mesectoderm of one 
density passes gradually into that of another. To supply these 
density relations which the model fails to give, one may refer to 
the sections. 
Fig. 2 does not represent the entire mesectoderm, but merely 
the connective tissue which remains after the nervous part of the 
primitive mesectoderm has been deducted. The model consists of 
eleven incompletely separated parts, belonging respectively to the 
anterior head region, and to each half of the five branchial arches. 
It is viewed in fig. 2 from the ventral surface. If one imagines the 
model thus seen revolved on its long axis until the lateral surface 
faces the observer, and then imagines the long axis inclined so that 
Morpholog. Jahrbuch, 25. 27 
