Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 547 
ency of its cells. The subsidiary layer (Pl 9e, fig. 6, , o') differs in nothing 
from that of the last embryo, (PI. 9d, fig. 1,) excepting that it is not separated 
from the vertebral layer (/1) by a hollow space; but this arises, as we have 
shown before, from the inequality in the development of the different organs. 
In another embryo, which in some respects is no farther developed than the 
last, that part of the spinal tube which forms the brain (Pl. 12, fig. 3, e', fig. 
3a, e') is closed over at the most anterior part; but the meeting edges are as 
yet not obscured, and a slight depression (e') remains at the extreme end of the 
fold, though it does not appear to amount to a passage-way into the brain cavity. 
The posterior part of the spinal marrow is much more extended backwards than 
in the last, and narrowed into a distinct band, still open and spreading at the 
hind end. The dorsal vertebrae are more marked, both externally and by a 
growing transparency in the centre, (Pl. 12, fig. 3, 3a, 3b,) and the backward 
extension of the vertebral layer is more defined and distinct from the subsidiary 
layer. We have already mentioned the wings (Pl. 12, fig. 3, 3a, a’, a®) which 
stand out on each side of the anterior part of the body, but again call the atten- 
tion of the reader to this pomt, in order to explain the singular appearance of 
that region im another embryo, of about the same age (PI. 12, fig. 4). Here 
the inequality of position and configuration of the projections that stand out 
from the body arise from the slight turning of the embryo upon its axis, and 
the consequent tension of the wings. 
In a phase a little farther advanced, (Pl. 12, fig. 7,) m addition to the increased 
closing over of the tube of the spinal marrow at the posterior end, the farther 
multiplication of the dorsal vertebrae behind, and the increased backward exten- 
sion of the anterior edge of the closing over ventral cavity, a new feature 
appears—a glimpse of which has been given in a younger stage, (PI. 9d, fig. 1, 
h, j?; Pl. 9e, fig. 5, 7?)—-upon the upper surface of the subsidiary layer, in the 
form of a broad, transverse band, (Pl. 12, fig. 7, 7) connected at its middle with 
a longitudinal one, (/,) which forks (j) as it extends towards the head. A closer 
examination shows that this band is a hollow tube, and contains a movable, 
granular fluid, indicating the first steps towards the development of the circulatory 
system. At this time the circulation is not continuous, but moves simply back- 
wards and forwards, in compliance with the impelling force of the periodical con- 
traction and expansion of the longitudinal portion of the tube, (/,) which is the 
heart, without doubt. Beyond the outlines of the body, this figure represents 
the dark, clear space (d@) mentioned in a former page, (p. 538,) and also the 
thickening of its outer edge (7') in the subsidiary layer. It will be noticed, that, 
on each side of the body, this thickened ring curves inward towards the trans- 
verse portion (7) of the circulating system within the body. The meaning of 
