916 
ME. J. LOCKHAET CLAEKE ON THE DEVELOPMENT OE 
In. foetal sheep of exactly 2 inches in length, it was found that the different parts of the 
spinal cord already described had become more or less modified in form, size, and relative 
arrangement, that new parts had been superadded, and that the structural elements of 
the grey substance had undergone considerable alterations. Fig. 4, Plate XLV. repre- 
sents a transverse section of the cord from the same region as before, viz. the upper part 
of the lumbar enlargement. Here we at once perceive that each lateral half of the grey 
substance is fashioned into very distinct anterior and posterior cornua [f, & I h), which 
are partially separated from their fellows of the opposite side by an anterior and a poste- 
rior median fissure (m, n), both of which were absent in the former sections, and now 
come into existence, as we shall presently see, as a consequence of the development of 
the cornua. We also observe that the anterior half of the central slit represented in 
the preceding figures has dilated into an oval canal (o), which is near the centre of the 
section. Moreover it will be seen that two new portions of the posterior white colunms 
(P’P) made their appearance on each side of the posterior median fissure. Let 
us now consider the course of the developmental growth by which these changes have 
been gradually effected. First, then, it may be remarked that in fig. 4 the anterior 
cornua are formed by a growth of the grey substance forward and inward, in front 
of the canal, from the imaginary line (/), which corresponds to the line of limit of the 
anterior grey substance (/) in fig. 3. The anterior white columns (A, A) extend in the 
same direction around the grooving ends of the cornua, until only a narrow space or 
fissure is left between their inner borders in the mesial line. In this way is formed the 
anterior median fissure (m) in front of the commissure, which projects into it as a conical 
process. During these changes in the anterior part of the cord, the posterior grey sub- 
stance {11)^ fig. 3) grows obliquely outward and backward J, fig. 4); while its ante- 
rior angle (at h) becomes further removed from the posterior angle of the anterior cornu 
(jA'), and is separated from it by a broader and deeper indentation, which is the rudi- 
ment of the neck or cervix cornu posterioris. This space or indentation is filled up 
and overlaid ufith the lateral column (A'), which has reached the level of the posterior 
column [c] and assumed a convex surface. This so-called lateral column differs from 
the rest of the white substance in being subdivided into a much greater number of smaR 
but separate fasciculi of various shapes, by means of a remarkable system of radiating 
fibres Avhich proceed both from the grey substance between the cornua and the epithe- 
lium surrounding the canal. A radiation of fibres, hoAvever, but of much less extent, 
takes place from the whole circumference of the grey substance. 
At the commencement of these changes, the central fissure or canal represented in fig. 3 
reaches the surface of the posterior grey substance. The growth of this substance is 
then continued not only backward, but outward, or divergent from the mesial line, while 
in the intervening angular and gradually increasing space betAveen it and this hne (fig. 
4, n) there are developed on each side two new pyramidal columns of longitudinal fibres 
[qj,])'), which increase in depth in a corresponding proportion. Of these, the outer one 
(2->), which is much the larger, rests on the back of the cornu, over which it ultimately 
