926 
ME. J. LOCKHAET CLAEKE ON THE DEVELOPMENT OE 
section of the upper part of the sacral enlargement of the chick, on the fifth day of 
incubation. The central dark layer (a) surrounding the canal is still uncovered by the 
posterior columns (c, c), and forms the posterior surface. The grey substance hitherto 
is not divided into distinct cornua ; there is no trace of a posterior median fissure, and 
only the first rudiments of an anterior (m). In the course of four days vs^e find that 
rapid and remarkable progress has been made in developmental growth. Fig. 20, 
Plate XVII. represents the appearances in a transverse section of the same part, at the 
end of the ninth day of incubation. By a succession of changes similar to those that 
have been described in the foetal sheep, both the anterior and posterior cornua have 
become fully developed, with corresponding median fissures. In each anterior cornu (/) 
is a thick cylindrical column of large nerve-cells. These cells, however, for the most 
part, differ both in shape and mode of formation, from those that are found in the corre- 
sponding part of the human and mammalian foetus. The majority are fusiform from 
before backward, and continuous with the antero-posterior fibres which reach the poste- 
rior cornu. They are not formed within large round and oval spaces with thick walls, 
like those already described in the mammal, but grow side by side in close apposition 
by the extension of substance from the ends of their nuclei, apparently after the manner 
of those in the central part of the grey substance of the Sheep and Ox, represented in 
fig. 16, Plate XLVII. Besides their antero-posterior processes, they send off others both 
outward and inward. The middle portion of the grey substance between this vesicular 
group and the posterior cornu contains a few isolated cells of the same kind. 
The antero-lateral white columns (h, h') have increased considerably in area, and the 
anterior median fissure between them is much deeper, while the canal (o) is reduced in 
size and limited to the centre of the section. The caput cornu posterioris (lb), on each 
side, consists of a dark mass of closely aggregated nuclei, which are smaller than those 
in any other part of the grey substance. It is entirely covered by the posterior white 
column (c ^), which, however, does not extend along the inner side of the cervix (q). 
The space between the cervix cornu (q) of one side and that of the other, and which, 
in the corresponding part of the adult cord of both the bird and mammal, is occupied 
by the inner and deep portions of the posterior columns, is now filled up by a bell-shaped 
mass of connective tissue (n'), which consists of a loose network of fibres connected at 
intervals with nuclei. The fibres of this tissue are also directly continuous with the 
connective tissue of the inner part (jp) of the posterior white column, and with the 
network in the cervix cornu (q), along the inner border ; but its fibres are coarser, and 
its nuclei are larger. Its deep portion, which is divided by a central raphe, forms the 
posterior wall of the canal (o), and constitutes its epithelium on that side; while its 
superficial portion extends on each side over the posterior white column (p), and is con- 
nected at its convex surface with the pia mater which surrounds the cord. In the lower 
part of the sacral enlargement (fig. 21, Plate XLVII.), the cervix cornu is united along 
the middle line with its fellow of the opposite side, and forms with it a single mass ; sc 
that only the extremities of the cornua surrounded by the posterior columns remain 
