OUTGROWTH OF THE NERVE FIBER 
793 
At this point it will be profitable to inquire a little more fully 
into the supposed syncytial nature of the central nervous sys- 
tem. When sections alone are studied, there may be an appar- 
ent justification for regarding the walls of the neural tube as a 
mass of protoplasm with nuclei embedded in it,^ for, as has already 
been pointed out, the cell boundaries within the medullary cord 
are difficult to make out unless they happen to be indicated by pig- 
mentation. When examined in the fresh condition, an entirely 
different state of affairs is revealed. It is astonishing how easily 
the cells, which in sections seem to be baked together in a mass, 
come apart when the medullary cord is dissected out of the living 
embryo and teased in water or salt solution. The cells appear as 
round glistening vesicles under the binocular microscope, and un- 
der the oil immersion they are found to be very clearly defined, 
each being surrounded by a very delicate, though perfectly dis- 
tinct, cell membrane. The cells are gorged with yolk granules, 
and the nucleus appears as a clear space near the center of each 
cell (fig. 15). There is not very much difference in the appear- 
ance of the cells in the different media named, though in water and 
the more dilute salt solution (0.2 per cent) there is some imbibition 
of water, which may result in the formation of a more or less 
eccentric clear zone just beneath the cell membrane (fig. 15 c). 
No sign of protoplasmic bridges can be made out. From these 
observations the conclusion seems clearly justified that the med- 
ullar}^ cord of the frog embryo is made up of perfectly distinct 
cells. It is in no sense a syncytium, and statements to the con- 
trary based upon the insufficient evidence from stained sections, 
are to be received with skepticism. 
The medullary cord is sharply marked off from all surrounding 
structures except where the ganglion crest is breaking down. 
The cord is in direct contact with the muscle plates and the noto- 
chord, but in the angle between the two latter structures, and in 
the the grooves between successive somites there are small spaces, 
which at this period are entirely devoid of cells. Just what is 
the structure of the material that fills these spaces in the living 
* Cf. for instance Weysse and Burgess (1906) on the histogenesis of the retina. 
