The white matter forms a transparent investment of
the grey matter; it arises as four patches, viz. an anterior
and a posterior white column on each side, which lie on
a level with the origin of the anterior and posterior
nerve-roots. It is always, at first, a layer of extreme
tenuity, but rapidly increases in thickness in the sub-


VIII. | THE GREY MATTER. 253


sequent stages, and extends so as gradually to cover the
whole cord (Fig. 88).
aew


SECTION THROUGH THE Spinal Corp oF A SEVEN Days’
CHICK.
pew. dorsal white column. Jew. lateral white column. acw. ven-

* tral white column. c. dorsal tissue filling up the part where

the dorsal fissure will be formed. pc. dorsal grey cornu.
ac. anterior grey cornu. ep. epithelial cells. age. anterior

commissure. pf. dorsal part of spinal canal. spe. ventral
part of spinal canal. aj. anterior fissure.
The grey matter and the central epithelium are
formed by a differentiation of the main mass of the
walls of the medullary canal. The outer cells lose their


254 THE FIFTH DAY. [CHAP.
epithelial-like arrangement, and, becoming prolonged
into fibres, give rise to the grey matter, while the inner-
most cells retain their primitive arrangement, and con-
stitute the epithelium of the canal. The process of
formation of the grey matter would appear to proceed
from without inwards, so that some of the cells which
have, on the formation of the grey matter, an epithelial-
like arrangement, subsequently become converted into
true nerve-cells.
The central epithelium of the nervous system pro-
bably corresponds with the so-called epidermic layer of
the epiblast.
