or 
Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 54! 
(Pl. 9d, fig. 1, e7; Pl. 9e, fig. 5, ce’) and the contiguous portion on each side of it 
(p). At about midway between the head and tail, this band is divided transversely, 
by four or five fissures, into so many block-like bodies (Pl. 9d, fig. 1, /; Pl. 9e, fig. 
5, f; Pl 12, fig. 1, 7, fig. la, f; Pl. 24, fig. 13, 7). These blocks are the dorsal 
vertebree; and through them we recognize that the “broad band” of former pages is 
the vertebral layer or basis, from and by which the backbones, constituting the 
axis of the skeleton, are formed. The chorda dorsalis along its middle portion 
is now distinctly separated from the vertebral layer, but remains hardly differen- 
tiated from the latter at its ends, (Pl. 9d, fig. 1, g, g',) and may be recognized 
as a long, cylindrical body, (Pl. 9a, fig. 1, gy, g'; Pl. 9e, fig. 5, g,) tapering before 
and behind, lying between the two halves of the broad band, to which it forms 
an axis. When seen through the thickness of the spinal marrow, this  stylet-like 
body resembles a long and narrow continuous band, (Pl. 9e, fig. 5, g-y,) bounded 
on each side by the abutting inner edges (f*) of the dorsal vertebrae (/) men- 
tioned above. 
The next lower and more interior layer, the subsidiary layer, (Pl. 9d, fig. 1, 
n; Pl. 9e, fig. 5, 7.) is separated from those above by a shallow, open space, 
(Pl. 9d, fig. 1, h, 7?; Pl. 9e, fig. 5, y?,) which extends as far as the length and 
breadth of the body, and is deeper at the cephalic end, where a_ considerable 
portion is almost isolated (Pl. 9d, fig. 1, 4) from the rest. It will be seen pres- 
ently that this is of significant importance in relation to the location and devel- 
opment of the system of circulation. The subsidiary layer, which forms the 
lower floor of this cavity, is thinner than formerly along its middle line, (PI. 
9d, fig. 1, ,) excepting at the cephalic end, where it thickens as it follows the 
backward folding of the germino-amniotic layer (Pl. 9d, fig. 1, a°). Beyond the 
outline of the body, however, it thms out again in all directions, and follows for 
a short distance the amnios ‘as it folds upwards, but soon leaves the same and 
takes a more direct course (a’, a‘) to the edge (d@) of the area pellucida (c). 
At the point where it leaves the cephalic hood to follow its own course forwards, 
the subsidiary layer makes a rather sudden bend, which, when seen from below, 
resembles the anterior edge of a broad wing, (Pl. 12, fig. la, a’,) whose posterior 
edge (a°) is the line along which the amnios, and along with it the subsidiary 
layer, projecting from each side of the body, bends upon itself, preparatory to 
covermg the head. In older stages, this is still more prominent (Pl. 12, fig. 3, 
gda,, a, 2°): 
There is, at this time, beyond the body and for a certain distance (Pl. 94d, 
0°, fi 
fig. 1, from d to 7; Pl. 9e, fig. 5, d to @) outside of the area pellucida, a dark, 
clear space of a strongly marked character, and readily recognizable by the naked 
eye. (Compare Pl. 14, fig. 12, which, although a little older phase, presents to 
69 
