Cuap. II. FORMATION OF THE ORGANS. 599 
the vascular area enlarges; but we do not believe that this occurs extensively, 
nor that the vascular area expands beyond a certain limit. By the time 
these walls are formed, the yolk begins sensibly to diminish in bulk, and the 
vascular area gradually folds around this shrinking mass; thus essentially cover- 
ing a greater proportion of its circumference, but by no means a wider extent of 
actual surface. Toward the end of the period of incubation it even diminishes 
in area, till finally it disappears altogether, within a few months after the Turtle 
is hatched. 
A peculiarity, which, until quite a late period, distinguishes the omphalo-meseraic 
arteries from the veins, is that they run, even to their outermost extremities, (PI. 
17, fig. 6, fig. 7,) im a very shallow stratum, the subsidiary layer, without plung- 
ing into the yolk mass beneath. Finally, however, they extend into the mass 
of the yolk, (Pl. 17, fig. 1,) and anastomoze with the branches of the omphalo- 
venous system (Pl. 18, fig. 4). The irregularity of development of the vascular 
area is particularly noticeable among the other inequalities of the development of 
the organs. Pl. 13, fig. 11, Pl. 14, fig. 6, 9, 11 are all drawn, on the 10th and 
llth of July, from eggs laid by the same Turtle, on the 18th of June; and 
yet, in Pl. 14, fig. 11 the vena terminalis is partially formed, in Pl. 13, fig. 11 
it is a little more advanced, in Pl. 14, fig. 9 it is complete, and the vascular 
area covered with bloodvessels; and in Pl. 14, fig. 6 the omphalo-meseraic arte- 
ries are still more numerous. Another, laid on the 13th of June, (fig. 8,) is not 
so far advanced as one laid on the 18th of June, (fig. 6,) whilst the latter is less 
advanced than one (fig. 7) laid on the 25d of June. Compare also Pl. 13, fig. 
4 and 5 with fig. 6, 7, 8, 9, all of which were laid on the 12th of June, and 
opened July 11th. Within the body, the changes in the omphalo-meseraic arte- 
ries are very simple. At first they are numerous, (Pl. 18, fig. 7,) and spring 
from along the whole length of the dorsal artery; but after a while they become 
concentrated within a certain area, (PI. 13, fig. 5, g,) midway between the head 
and tail, and finally merge into a single thick vessel, (Pl. 18a, fig. 14, 7) which 
branches beyond the body as it passes into the vascular area. It soon begins 
to elongate, at the same time lessening in diameter, (Pl. 18a, fig. 8, 7° to J 
fig. 9, 7%,) and passes down between the anterior (fig. 8, n’, fig. 9, n’) and pos- 
terior (fig. 8, x', fiz. 9, 2!) limbs of the curve of the intestine, where the latter 
protrudes beyond the body, in the shape of a U; it then forks, one branch pass- 
ing on the right, and the other on the left, of the curve. Each branch of the 
fork divides again, almost as soon as it leaves the main vessel, and these branches 
give rise to others, and these to still others, and so on until they become very 
numerous. At last, the efferent artery gives off branches, within the body, which 
ramify extensively over the intestine (Pl. 17, fig. 1). 
