The Development of the Cartilaginous Skull etc. in Necturus. 395 
with yolk granules at the right of this mesothelial tissue and in the 
walls of the vascular spaces form the mesenchyma of the section. 
Most of the cells of the mesectoderm lie between the vascular spaces 
and the wall of the anterior branchial cleft, a few cells, however, 
are seen at the left, just beneath the ectoderm, and near the meso- 
thelium of the arch. 
The section shows filaments of protoplasm, but few, if any, cell 
outlines are to be seen in the median tissues, and the number and 
position of the cells may be determined merely from their nuclei. 
The separation of the two median tissues is sharper in the mandi- 
bular or hyoid arches, where mesenchymal cells of the vascular 
system are not interpolated in the mesectoderm. Fig. 15’ does not 
show the most marked difference between the median tissues, but 
rather illustrates the manner in which the two tissues become entangled 
in the vascular region of the gill-bearing arches. 
I have been accused of maintaining that the absence of yolk 
granules in the mesectoderm proves this tissue to be of ectodermic 
origin. Lest others than my accuser should also believe the accusa- 
tion warranted, I take the present opportunity to say that I know 
the mesectoderm of Necturus to be of ectodermic origin 
because I have carefully followed the tissue from its origin, 
tracing the development through slight degrees of growth, 
in embryo after embryo, from the very beginning until the 
stage now described. The marked differentiation of tissues 
occasioned by difference in the size and quantity of the yolk granules, 
though proving nothing in itself, makes it possible to distinguish the 
median tissues of Necturus from one another with greater certainty 
than that with which they may be distinguished in any other Ver- 
tebrate embryo with which I am acquainted, although I am told that 
a similar differentiation occurs in other of the Amphibia. 
The colored figures are not given to show that part of the 
median tissues is ectodermic, while another part is not. They are 
given that those who may not be able to examine Necturus embryos 
for themselves may see how apparent the difference between the 
two kinds of tissue really is. 
Fig. 1¢ passes through the infundibulum and the dorsal part of 
the optic vesicles. The hyomandibular pocket approaches the ectoderm 
at the left of the section. and touches the ectoderm on the right. The 
fifth branchial cleft has passed out of section, and of the fourth cleft 
only the ventral wall is seen. The compact group of mesoderm cells 
