The Development of the Cartilaginous Skull ete. in Necturus. 385 
facts of development, if it were further assumed that certain pro- 
perties, usually developed in the equatorial cells (ex. skeletal) are 
potentially properties of the cells at the ectodermic pole of the egg, 
while excluded from the cells at the endodermic pole, and that other 
properties of the equatorial cells (ex. vascular) are potential properties 
of the cells of the endodermic pole, but not of the ectodermic. 
Such a theory accounts for the inability of one germ-layer to 
replace another, since no two are in all respects potentially equivalent. 
It also seems to meet the conditions of actual development better 
than the long accepted view that each germ-layer differs absolutely 
from the others in the kind of tissue it can produce, or than the 
more recent theory that any tissue may arise from any germ-layer. 
The association of endodermie cells with the mesoderm in the form- 
ation of vascular tissue seems to me. significant in consideration of 
the fact that in lower organisms circulatory functions belong to the 
digestive tract, and have been assumed by the mesoderm during 
the evolution of higher forms. A similar reason for the association 
of ectodermic cells with the mesoderm in the formation of supportive 
or protective tissues suggests itself, since of the two primitive germ- 
layers, it was doubtless the ectodermic which originally fulfilled these 
functions. 
However, before any theory can be accepted as adequate, we 
need to know, as BARFURTH suggests, whether the smooth muscle 
fibre produced by the ectoderm is in every way similar to that 
derived from the mesenchyma; we need to know whether cells which 
leave their epithelial connection and wander into the mesenchyma 
lose or retain the primitive characteristics of the layers from which 
they are derived. In short, we need to know the origin and fate 
of all of the cellular elements of which the mesenchyma is composed. 
The first effort to determine what becomes of the cells of the 
neural crest after they enter the mesenchyma came from GORONO- 
WITSCH (92) in a preliminary paper, entitled »Die axiale und die 
laterale Kopfmetamerie der Vogelembryonen. Die Rolle der sog. 
‘Ganglienleisten’ im Aufbau der Nervenstämme«. In this paper 
GORONOWITSCH notes that the cells of the neural crest become 
scattered as they approach the dorsal part of the axial mesoderm, 
with which they soon absolutely identify themselves, so that the two 
kinds of cells cannot be distinguished (page 456). One can only say 
in general that the elements of the scattered neural crest group 
themselves in a region dorso-lateral to the axial mesoderm, from 
Morpholog. Jahrbuch. 25. 26 
