410 Julia B. Platt 
(94), and know to be ectodermic, yet I doubt if it be possible to 
convey to others by means of a few sections selected from two or 
three series, the conviction one feels who has studied entire series 
from nearly two hundred embryos in successive stages of develop- 
ment. As the mesectoderm is not shut off from the mesenchyma by 
a limiting membrane, the cells composing the two tissues come into 
immediate contact, and the possibility that some cells from the 
mesenchyma lose their yolk and join those arising from the ecto- 
derm cannot be excluded, any more than such a possibility ean be 
excluded in the case of the cranial or spinal ganglia, which also 
are not. at first sharply separated from the surrounding mesenchyma. 
One can only say that there is no positive evidence that such cells 
do add themselves to the mesectoderm, as might be furnished, for 
example, by an intermediate condition in the appearance of the cells 
along the line where one tissue meets the other. Moreover, the 
comparison of the relative number of cells in the two tissues, and 
of their grouping, in closely successive stages demonstrates absolu- 
tely that if there has been any addition to the mesectoderm of cells 
from the mesenchyma, they have been very few in number. 
It should be remembered that in the earliest stage in which 
wandering cells comparatively free from yolk are found, these cells 
lie in the dorsal and dorso-lateral part of the head, and that later 
such cells are not found in this region, save the relatively small 
number taking part in the formation of the nervous system, but are 
found in the ventral part of the head, while intermediate stages 
show the cells in intermediate positions. 
The Procartilage in Necturus 15 mm long. 
I now pass to the description of the branchial arches in an 
embryo of 15 mm. No true cartilage has yet appeared, but the 
primitive position and shape of each cartilaginous bar is definitely 
determined by compact masses of cells, whose nuclei surrounded 
by very little protoplasm compose a peculiar tissue called »procarti- 
lage«. The superficial cells of the prochondral bars thus formed, 
are in places flattened against the body of the bar, showing that 
a perichondrial differentiation begins to be established. This happens 
especially where but few mesectoderm cells intervene between the 
surface of the bar and the neighboring mesodermic or endodermic 
tissues. In such regions, the beginning of a perichondrial layer 
seems to show that the growth of the prochondral bar by accretion - 
