The Development of the Cartilaginous Skull ete. in Necturus. 889 
assumes that they form with the mesenchyma a homogeneous tissue. 
To my thinking inability to distinguish cells from the two sources 
by no means proves their identity, but should it be possible to dis- 
tinguish the tissues in other Vertebrates, as in Amphibia or in 
Petromyzon, the inference is rather that a difference actually 
exists in forms where the cells appear, with our present methods of 
differentiation, absolutely alike. 
Two interpretations of the phenomena connected with the de- 
velopment of the branchial cartilages in Necturus suggest them- 
selves. 
One may hold with KAsTscHENKo (loc. cit.) that the mesenchyma 
is a tissue composed of cells left over from the formation of epithelial 
tissues by any of the germ-layers, and that it is a matter of indiffe- 
rence which of the mesenchymal cells take part in the formation of 
any tissue, or whence the cells are derived. As favoring such a 
view one may regard the fact that the cartilage of the skull, in- 
cluding the posterior part of the trabeculae, although of mesodermic 
origin, apparently forms a homogeneous tissue with the cartilage of 
the branchial arches and of the anterior part of the trabeculae, 
which is of ectodermic origin. The fact that true bone arises from 
mesodermal cells, while the primitive dentine is derived in Necturus 
from the mesectoderm looks in the same direction. 
On the other hand, were it a matter of indifference which cells 
of the mesenchyma form a given tissue, it is not easily explicable 
why the mesectoderm, which arises in the dorsal and dorso-lateral 
part of the head, should migrate en masse towards the ventral part 
of the head, while mesenchymal cells of mesodermie origin migrate in 
the opposite direction. As the two tissues pass one another, the 
mesectodermic cells lie to the outside, the mesodermic to the inside, 
and each tissue appears to maintain its independence. Those mes- 
ectodermic cells which compose the connective tissue thus appearing, 
not less than the cells which compose the peripheral nervous system, 
to have a definite purpose before them — a purpose which requires 
a longer journey by a greater number of cells than is demanded 
by the peripheral nervous system of the embryo in this early stage 
of development. | 
A second view which may be held, is that the branchial cartilages 
and the anterior part of the trabeculae are not only ontogenetically, 
but also phylogenetically structures of ectodermie origin, possibly 
constituting with the notochord the primitive skeleton of the ancestral 
