Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 549 
heart (Pl. 12, fig. 9, 2) has bent upon itself slightly, assuming a sigmoid flex- 
ure, so as to produce a protuberance in this part of the body. The large 
vessel, spoken of above, in a little younger stage, as a broad, transverse band 
(see p. 547; Pl. 12, fig. 7, ¢) which is connected with the posterior portion 
of the heart, is now bent sharply upon itself, so as to assume a furcate shape 
(Pl. 12, fig. 10, “). The heart (4) has but a slight flexure laterally, but is bent 
strongly upon itself as it follows the curvature of the body to join the forked 
vessels (7) which run towards the head. The clear, dark, circular area, which 
we have already mentioned above (Pl. 12, fig. 7, d) as surrounding the embryo, 
still preserves a homogeneous aspect, (PI. 14, fig. 12,) but has expanded consid- 
erably, so as to exceed the embryo by one half the length of the latter. 
Proceeding to a still further advanced phase, we find the embryo strongly 
bent upon itself vertically at each end, (Pl. 12, fig. 10,) so as to bring the 
head down towards the heart, and the tail towards the abdominal cavity. The 
tube of the spinal marrow is nearly altogether closed over (Pl 12, fig. 10, é, 
é, 10a, e, 10b, ¢, 10c, e). The apparent gaping in one of the figures (fig. 10a, 
e) is owing to the circumstance that the view is not taken from the outer sur- 
face, but from the deeper parts of the spinal tube, thus showing the hollow pas- 
sage through and along the organ. The dorsal vertebrae (Pl. 12, fig. 10, f, 10a, 
‘f) are more clearly defined. The eyes and ears are not so prominent as in the 
last-mentioned phase. The subsidiary layer, following the curvatures and the 
approximating sides of the lower surface of the body, has become a sac, whose 
broad mouth opens downwards through the abdominal aperture (0) and in contact 
with it. Thence it spreads out, as heretofore, over the parietes of the yolk mass. 
The heart, (Pl. 12, fig, 10, %,) in this animal, appeared still as it has already 
been described before; but, in continuance of this part of the subject, we may 
now point out the peripheric extension of the circulatory system. The clear, 
dark space bounded by an annular thickening of the subsidiary layer, (Pl. 12, 
fiz. 7, 7,) which has already been described, (p. 547,) is more or less streaked 
here (Pl. 13, fig 10; Pl 14, fig. 11) by dark, transparent channels, that run 
radiatingly from the embryo towards the parietes of this transparent area, where 
a few of them join a broken, irregular circle (fig. 11) of similar channels. The 
ring bending towards the body, which was pointed out in a former stage (Pl. 
12. fig. 7, 7!) as curving in close connection with the transverse broad band, or 
vessel, (i) behind the heart, corresponds here to the circular channels, which also 
curve inwards towards the same region as above, and join the transverse, or 
rather now forked, vessel, (Pl. 12, fig. 10, 7) which enters the posterior end of 
the heart (/). Within all these channels, so far as they are directly in connec- 
tion with the heart, there is a backward and forward motion of a granular, 
