Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. A435 
more prominently the growing differentiation of the two opposite portions of the 
embryonie dise known as the cephalic and the caudal amniotic hoods, in contra- 
distinction to those parts situated between them and at right angles with them, 
another feature develops itself, under the form of a sharply marked, superficial, 
short furrow, beginning near the cephalic end, (PI. 9e, fig. 3, 4; Pl. 11, fig. 3, 4,) 
and trending thence backwards; thus dividing the body into two equal portions, 
which correspond to the right and left halves of the whole system. The elonga- 
tion of this furrow continues for some time, (Pl. 11, fig. 4, 5, 6; Pl 9e, fig. 
4, b,) and reaches almost to the caudal end, before any other peculiarity in any 
way related to its growth appears at the surface. A transverse section of the 
embryo, at the two periods just mentioned, shows that the longitudinal furrow 
(Pl. 9e, fig. 3a, 4, 4a, 2.) is not a hollowing out of a channel in the depth of 
the germinal layer, but that the whole thickness of the latter is depressed in 
the form of a sharp fold. which projects like a ridge on the lower side. But, 
beneath all this there is an important change going on, which has the closest 
connection with the furrowing above. In the younger of the two embryos just 
quoted, (Pl. 11, fig. 3; Pl. 9e, fig. 3, 3a,) at the upper side of the thick sub- 
sidiary layer mentioned above, (p. 536,) a broad band of its component, loosely 
packed, granule-like cells, as thick as the germinal layer above it, has become 
separated and combined into a more firm stratum, (PI. Je, fie Sift, Page ify) 
equalling the length, and following the curve, of the whole embryo, but falling 
considerably short of the breadth of the same, and only occupying about one 
third of its whole breadth on each side of the middle line or furrow. The ante- 
rior and posterior ends (Pl. 9e, fig. 3, f1) of this band still remain continuous 
with its original basis, the subsidiary layer, (Pl. 9e, fig. 3, 0',) but laterally it 
is clearly and sharply separated (PL Qe, fig. 3a, f°). It is important to notice, 
that, in this instance and in others of a similar kind, this proceeding is not a 
splitting up into thinner membranes of an already well developed cellular tissue, 
but a secession, a withdrawal of a certain amount of loose, unconnected cells 
from a larger bed or heap of the same materials. However, inasmuch as these 
cells are evidently arranged for a certain purpose, we do not intend to deny that 
there is a determined relation among them, though still in an incipient state. 
In the older embryo, noticed above, this broad band still follows the curvature 
of the lower side of the body, (Pl. 9e, fig. 4, /%,) which, up to this time, is 
greatest, and more folded in, at the cephalic end, whilst it has become more extended 
laterally (fig. 4a, /', f°). Its central axis beneath the furrow appears to be 
differentiated in structure, since, when seen from below, it presents a sharply 
defined, narrow strip, having a tint dissimilar from that on each side of it (PI. 
11, fie. 5a, 4, and Pl. 9e, fig. 4a, gy). This is doubtless the chorda dorsalis, the 
° co] 
