DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 537 
from the skin and neural tube, but chiefly formed by the 
protoplasmic prolongations of the yolk-laden reticulum of the 
mesenchyme. Into this reticulum the nuclei of the neural 
crest, as Sedgwick tells us, migrate. But in the head of 
Necturus the protoplasm of the neural crest is filled with 
small yolk granules, and here again the nuclei take their 
surrounding protoplasm with them. A common reticulum, 
therefore, does not exist. 
Is the cell, then, a separate unit, or merely the node of a 
reticulum? In Necturus many cells of the neural crest mi- 
grate from the dorsal wall of the brain to the branchial arches, 
while cells arising in the wall of the archenteron migrate up- 
wards until they lie between the dorsal wall of the brain and 
the skin. I have not used microscopic methods that enable 
me to state from observation that a fusion of the protoplasmic 
threads passing from cell to cell does or does not take place. 
But if such a fusion exists, it is continually renewed to be 
immediately interrupted, since the nuclei in migrating past 
one another take their “nodes” with them. That they do 
this is most clearly evinced in the formation of the aortic 
arches, where individual cells of endodermic origin migrate 
with their large yolk granules through surrounding mesecto- 
dermic tissue from which the yolk granules have nearly dis- 
appeared. This constant association of a particular bit of 
protoplasm with a particular nucleus makes the existence of 
the separate and distinct cell highly probable, despite the 
reticular structure of the mesenchyme. 
I must also dissent from Sedgwick’s statement that cells of 
the neural crest give rise to the walls of the vascular system 
and to muscular tissues. It is, however, true that in Necturus 
cells of the mesectoderm are converted into blood- 
corpuscles. While the majority of the blood-corpuscles are 
derived from the endoderm, the mesectoderm of the branchial 
arches is also a source of their formation. 
The path of the ophthalmicus profundus is originally occu- 
pied by cells that, like those of the cranial ganglion, result 
from the fusion of the neural crest with the ectoderm. The 
