On 
Or 
eo 
Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 
is more developed. The brain (¢’)’ is already slightly three-lobed; the region of the 
medulla oblongata (e') is more closed over; the eye (/) is more isolated from the 
surrounding parts: the heart is much more enlarged, and stands out conspicuously 
beyond the ventral surface of the body; the aortic bulb (/') is now quite a prom- 
inent feature. At a point (j°) a short distance behind the heart, the dorsal artery 
(y*) gives off only a single, but very broad, omphalo-mesmeraic vessel, (j*,) as if the 
many vessels of the last phase had merged into one great channel. As this figure, 
with the exception of the head, is not an exact profile, but rather a combination of 
a profile view and a view obliquely from above, the dorsal artery (j*) appears to 
be nearer to the ventral surface than it really is, and the dorsal vertebre (/) 
seem thicker than is natural. 
In a little more advanced phase, (Pl. 15, fig. 2, and 3,) the spinal marrow is 
more extended backwards into a distinctly developed tail (wood- Woodentl: 
eut 1)2 The dorsal vertebrae (wood-cut 1, f) reach to the root Pn ite 
of the tail. The eyes have become entirely inclosed within com- = 
plete orbits. The heart has become three-chambered ; the single 
auricle of the last phase (Pl. 18a, fig. 14, 4) being now divided 

Sy 
i 
into two cavities, thus leaving its fish-like character, and adopting 
a structure which is peculiar to scaly Reptiles alone. The dorsal artery (wood- 
cut 1, j*) extends to the end of the tail, and the cephalic artery branches exten- 
sively in the head. The fork of the vena afferens, which in the beginning orig- 
inated close to the heart, (Pl. 12, fig. 7, 2 2 fig. 10, 2) but later receded (PI. 
18a, fig. 13, 7, 7) from this organ, is now (Pl. 15, fig. 2) some distance beyond 
the body, towards the vena terminalis. This embryo presents a feature (wood-cut 
1, n°) in the posterior region, which, at first sight, might be mistaken for the hind 
foot just budding forth. Upon close scrutiny, however, we discover that this pro- 
tuberance is in the perpendicular plane of the axis of the body, and is based 
upon that area close to where the tail arises. This protuberance is a simple hol- 
low sac, formed by the folding together of the two opposite halves of the sub- 
sidiary layer, uniting their edges below. The dorsal artery (wood-cut 1, 7?) runs 
close upon and above this protuberance, thus showing that the latter is a later 
production of the same layer which developed the former, the dorsal artery, upon 
its middle line. Since we find this protuberance (wood-cut 1, 2°) in such a con- 
nection, and moreover see traces of bloodvessels coming from the dorsal artery 
(wood-cut 1, y*) branching in it, we conclude that this must be the allantois. 
1 In Pl. 18a, fig. 14, the first letter to the left of in Pl. 13, fig. 2 which it represents, and is chiefly in- 
k, should be e*, instead of e!. tended to point them out more minutely, no letters 
* This wood-cut corresponds exactly to the parts having been introduced in the original figure. 
70 
