600 EMBRYOLOGY OF THE TURTLE. Parr III. 
The vena terminalis, as has already been shown, develops first im its peri- 
pherie portions, and finally joins the vena afferens, which at the time is a trans- 
verse and very short vessel (see Pl. 12, fig. 7, 7). The vena terminalis grad- 
ually embraces a larger area, up to a certain extent, (Pl. 13, fig. 2, fig. 12,) 
and then begins to sink below the surface of the yolk, (Pl 14, fig. 2, 2a, 
r', 7,) following the vena afferens, until, finally, it is lost in the maze of anas- 
tomozing vessels which empty into the vena afferens (PI. 9c, fig. 3). At the 
same time, however, the vascular area continues to expand superficially, until, at 
last, it embraces the whole yolk mass. At first, the vena afferens apparently lies 
within the body of the embryo (Pl 12, fig. 7, 7); but it is really as much 
without as within, being at the bend (corresponding to the point a® in fig. 1, 
Pl. 9d) where the vascular area joins the body. This transverse vessel (Pl. 12, 
fig. 7, % 7) corresponds to the fork of the vein, there bemg as yet no interme- 
diate portion between it and the heart (4). Soon, however, it assumes a furcate 
shape, (Pl. 12, fig. 10, 4) and a little later it retreats from the heart, becoming 
still more decidedly forked, (Pl. 18a, fig. 13, 7, 7) and empties into a short but 
distinctly developed single vein, the vena afferens (fig. 13, 7)" The vena affer- 
ens elongates rapidly, and its fork retreats still further, till it comes in a line 
with the vena terminalis (Pl. 18, fig. 7). After a short time it begins to sink 
into the ‘yolk, (PL 14, fig. 2, 2a, 7,) and then branches very extensively, until in 
time it passes directly (Pl. 16, fig. 2a, 7’, fig. 2b, 7°) through the vitellme mass, 
and the latter becomes filled by anastomozing vessels, (Pl. 9c, fig. 3; Pl. 17, fig. 
3, fig. 7,) constituting, not as before, a superficial vascular area, but a globose, vas- 
cular plexus, an extension of the vascular area, throughout the whole breadth 
and depth of the yolk. 
The Abdominal Vein. (See p. 552, and Pl. 18, fig. 7, etc.) 
The Allantoidian Artery. (See p. 553, Pl. 13, fig. 2, and w-ce. 1, n°, etc.) 
The Allantoidian Vein. (See p. 554, Pl. 13, fig. 2, and w-e. 1, 7, ete.) 
The Branchal Fissures. (See p. 548, and Pl. 12, fig. 8, m, 9, m, etc.) 
The Intestine. The digestive system begins rather late to develop, but its 
foundation is laid by the subsidiary layer (Pl. 9d, fig. 1, ”) quite early, in the 
form of two blind sacs, one just behind the heart, (4,) and the other, at the 
opposite end of the body, immediately inclosed by the incurved tail. As the 
abdominal aperture grows narrower, these sacs consequently increase in length, with- 
out undergoing any other change for some time. Of the two, the posterior one 
first loses its simplicity by the development of a hernia, the allantois, (Pl. 13, fig. 
1 The letter 7, nearest to the letter A® in fig. 13, 2 For further details upon this point, see p. 527, 
Pl. 18a, should be 7. 528, and 529. 

