598 EMBRYOLOGY OF THE TURTGE. PART UU 
fiz, 2; w-e. 1, 2°, p. 553) also arise from its posterior end. After a while its 
abdominal portion has less numerous outlets, (Pl. 13, fig. 3, ¢,) all of which finally 
merge into one, the omphalo-meseraic artery (Pl. 18a, fig. 14, 7%, 7°). This is 
very easily understood, if the very loose connection of the cells of the subsidiary 
layer is borne in mind; the channels for the blood, having no wall, readily 
change their course, and merge into each other, and finally form one large stream 
for the exit of the arterial blood. Around this single, large channel a distinct 
wall is eventually formed. 
In the beginning, the omphalo-meseraic artery, or rather arteries, are mere 
lateral diverticula of the dorsal artery, (Pl. 18, fig. 7,) but do not originate from 
it as a starting point. It is rather singular, but nevertheless true, that the first 
indications of this system of vessels appear at the extreme edge of the subsidiary 
layer, inthe form of a thickening (Pl. 9d, fig. 1, 7, 247 Ply 9e) figs: 5, #2) 24) 
of the periphery of the latter. This thick border goes on broadening for a 
while, till it becomes quite conspicuous (Pl. 12, fig. 7, 7’) as a well defined ring 
surrounding a broad space bordermg on the abdominal region of the embryo. 
One portion of it, that nearest to the head, curves inward, and lies in close 
connection with the transverse vessel (7) which leads into the posterior end of 
the heart. For a short time, this ring gradually expands, and incloses a large 
area, (Pl. 14, fig. 12,) without exhibitmg any other change; but finally little, dark, 
hollow spaces appear, (Pl. 14, fig. 11,) arranged in one, two, or three irregular 
rows or concentric circles. These are collectively called the vena terminalis; but 
it is impossible at this stage to separate them from the arterial vessels which 
commingle with them here, so that it is quite proper to say that the omphalo- 
meseraic arteries, and the veins also, originate first at the circumference of the 
vascular area. 
Almost synchronically with these circular vessels the true omphalo-meseraic 
arteries appear as dark streaks, more or less continuous, converging from the 
vena terminalis toward the body. Presently both the interrupted channels of the 
vena terminalis and the external omphalo-meseraic system anastomoze completely 
with each other, (Pl. 18, fig. 5, 7,) and form continuous channels for the circu- 
lation of the blood. The channels are very irregular at diverse points, (Pl. 18, 
fig. 6,) both as regards breadth and depth, some appearing very shallow, as if 
just forming. In the latter case, there is no mistaking that they are hollows in 
the upper surface of the subsidiary layer. It is difficult to say at what stage 
walls are formed around these channels; but certainly not till some time after 
they originate (PI. 14, fig. 5; Pl. 17, fig. 5). Finally, however, they are endowed 
with a very thin wall (Pl. 18, fig. 2). At the first glance it would appear as 
if the walls of the vessels must be constantly resorbed, and new ones formed as 


