Irving Hardesty. 89 
parallel and so thick as to obscure their individuality under the micro- 
scope. Intermingling with the nerve fibers along their course, these 
processes rapidly decrease in size as they pass inward with the result 
that areas in the periphery of the white substance are more richly sup- 
plied with neuroglia fibers than those farther in. 
The marginal veil is interrupted by no appreciable ingrowths of the 
pia mater except those accompanying the blood-vessels. Figure 1 is 
given to illustrate a locality showing this. The blood-vessel (artery, in 
this case) leaving the pia mater, carries with it an investment of pial 
tissue which, together with the tissue of its own walls, stains brownish- 
red (f). In passing through the marginal veil the blood-vessel appears 
to take with it an additional investment of neuroglia fibers (nm). ‘This 
neuroglia investment remains continuous with the marginal veil and 
though gradually decreasing, is maintained as far as the blood-vessel can 
be followed into the specimen—in some instances to the substantia 
erisea itself. It has been frequently observed that the neuroglia fibers 
are especially abundant about the blood-vessels, occasionally even reach- 
ing into their walls. This behavior has been considered as having to 
do with processes of nutrition. The well-differentiated preparations I 
have examined, both of man and the elephant, show no neuroglia fibers 
actually reaching into the walls of the blood-vessels and whether their 
greater abundance about the blood-vessels has to do with nutrition or 
not, the appearances suggest that their presence is the result of neu- 
roglia tissue having been carried in from the marginal veil by the blood- 
vessels as they pass through it. Therefore, if for a physiological pur- 
pose, their presence is due to a physical process. The blood-vessels 
grow in for the most part during the early stages of development, and 
at a time when the marginal veil, and the whole organ, is in a more 
plastic condition than it is after the nerve fibers grow in and the pro- 
cesses of medullation have begun. No neuroglia fibers are formed at 
the time the blood-vessels begin to enter and the neuroglia tissue carried 
in from the (then relatively thick) marginal veil simply remains about 
the blood-vessels and later develops into the adult form of neuroglia. 
This development is no doubt facilitated because of the nutritive condi- 
tions afforded there. 
In the early stages of growth the marginal veil (Randschleier of His) 
in man is described as formed of the fused distal ends of the epithelial 
(later ependyma) cells lining the central canal. At first it contains no 
nuclei but appears as a sort of reticulated protoplasmic continuum, the 
fusion of the cell processes having resulted in an obliteration of all cell 
boundaries. Later, as the walls of the. neural tube thicken still more, 
