914 
ME. J. LOCKHAET CLAEEIE ON THE DEVELOPIVIENT OF 
similar fibres which radiated forward from the epithelium (a) surrounding the front of 
the canal. At its opposite or anterior border — or rather anterior part, for it had no 
distinct border — this nucleated tissue became directly continuous with that which formed 
the body of the vertebra (JJ). This body consisted of a dense mass of closely aggre- 
gated nuclei, similar at its circumference to those of the connective tissue, but somewhat 
different in shape toward the centre, where they were round, oval, elongated, angular, 
and often curiously crescentic. They were all connected by fibres which, nearer the 
circumference, where the structure was looser, could be very easily distinguished; but 
in the centre they were less distinctly seen between the densely-crowded nuclei. Near 
this centre was a circular spot (^), the section of a longitudinal cylinder, suri’ounded by 
a thick wmll resembling the wall of a cartilage-cell and enclosing several closely aggi’e- 
gated nuclei, connected by short fibres. These nuclei were round, oval, ppiform, or 
otherwise irregular in shape, and rather larger and more granular than those by which 
they were surrounded. 
The spinal cord and vertebral column of the Sheep at this period of development have 
a considerable resemblance to those of the chick on the fifth day of incubation. In the 
latter, however, the different parts of the grey substance are more distinctly marked. Fig. 
1 9, Plate XLVII. represents a transverse section of the vertebral column and spinal cord, 
in the sacral enlargement of the chick, on the fifth day, and magnified 60 diameters. 
The section of the cord was nearly a perfect oval. There was no trace of the rhom- 
boidal sinus or ventricle which in the adult bird separates its posterior lateral halves*. 
The epithelial layer (a) immediately surrounding the canal was clearly defined and easily 
distinguishable from the rest of the grey substance. It was somewhat broader behind, 
where it reached the surface in the form of an arch. On each side of it the posterior 
grey substance (d) consisted of a pyriform mass, nearly covered by the rudiments of the 
posterior white column (c), and separated by a triangular and much lighter space from 
the anterior grey substance (f), which had a nearly quadrangular form, but was sepa- 
rable into two different masses, of which the outer was greyer and of an oval shape. 
From both of these the anterior nerve-roots (^) originated and escaped from the anterioi’ 
and outer angle of the grey substance. Along the posterior half of the epithehal layer 
(a), its nucleated network was arranged in a connected series of arched radiations, which 
extended into the pyriform mass of grey substance on the same side (d). From this 
substance another very evident series of separate fibres ran directly forward to that part 
of the cinterior grey substance {f) from which its nerve-roots chiefly arose. Neither 
these nor the arched fibres just mentioned as radiating from the epithelium could be 
seen so distinctly in the Sheep. Although the grey substance consisted of the several 
separate masses above described, yet there was but little diversity in the appearance of 
its structural elements. Its nuclei, generally, were rather smaller than in the section 
of the foetal sheep represented in fig. 1. 
The cylindrical column {h) in the centre of the body of the vertebra and corre- 
* See Philosophical Transactions, 1859, Plate XXIII. figs. 34, 35, and 36. 
