Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 555 
H®, ht; Pl. 24, fig. 7, 2°, #*) is not sensibly changed; but the dorsal artery (PI. 
18a, fig. 11, 7?) has increased in diameter, especially in a_ horizontal direction. 
Just behind the heart and close to the ventral surface there is a small, dark, 
round body, which, in all probability, is the imeipient liver. The allantois (Pl. 24, 
fig. 15, n°) has considerably increased in size, and is transformed imto a two-lobed 
organ, by a longitudinal constriction. From the region of the heart to the basis 
of the allantois, the subsidiary layer (Pl. 24, fig. 14, , fig. 15, 2) remains broadly 
open, but not to the extent that obtained in the last phase. The aperture of this 
layer is limited by the contracted sides of the body, which have reduced the abdom- 
inal opening (Pl. 24, fig. 14, 0) to one quarter the length of the body. The 
branchial fissures (Pl. 18a, fig. 11, m; Pl 24, fig. 7, m) are more or less gaping. 
On the exterior of the body, at a short distance behind the heart, and also just 
posterior to the allantois, the musculo-cutaneous layer projects in the form of lit- 
tle bud-like execrescences (Pl. 24, fig. 14, w'). There are a pair of these in front 
and a pair behind, corresponding to the position of the feet. 
Progressing still farther, we find an embryo with the head still more bent 
upon the thorax (Pl. 14, fig. 5) than in the last, so as almost to touch the heart ; 
this embryo exhibits also a considerable increase of size. The brain (Pl. 24, fig. 
9a, ¢, €*, &) is more deeply folded, especially at the sides (¢). The eyes (fig. 
9a, hk, fig. 12, &) are perfectly round, and very prominent; the pigment layer is 
quite black. The front end of the head is indented, close to the lateral parietes 
on each side of the middle line, by a deep, broad depression, (Pl. 24, fig. 12, 2,) 
the lower side of which is bounded by a diverticulum-like protrusion from the infe- 
rior surface of this region. The dorsal vertebrae are more numerously developed in 
the tail (Pl. 14, fig. 5). The heart (PI. 24, fig. 9, #°, A, fig. 9a, A*) is much enlarged, 
and projects so far on the abdominal side, that it renders this region the thickest 
part of the body; its auricles (fig. 9, 2’) are very distinct from the ventricle, 
(A‘,) not only by their position, but also by the peculiar spongy nature of the 
cavity of the ventricle (fig. 9, 4%, fig. 9a, 2+). The network of peripheric blood- 
vessels in the head is more numerously developed, especially at its end (Pl. 24, 
fig. 9a). The fork of the vena afferens (Pl. 14, fig. 5) is now at the vena 
terminalis; the ven abdominales (PI. 24, fig. 9, 2?) and the venx capitis (7°) 
empty with larger channels into the heart (4°). The abdominal veins (fig. 9a, 7°) 
are closely approximated to, but run in a plane a little below, the dorsal artery 
(j”). The omphalo-meseraic arteries (Pl 14, fig. 5) spring from one common 
point of departure, and, by their increased size, render the vascular area much 
darker. The subsidiary layer has undergone a considerable change at the ante- 
rior part of the body, both in front and above the heart, where, in the form 
of a great, capacious tube, (Pl. 24, fig. 1, I, 1’, 1’,) it forms the lining of the 
