Cuar. IL. FORMATION OF THE ORGANS. 597 
pa, 10a, pa, 10b, pa) aortee and the pulmonary artery (fig. 10, 10a, 10b, /', 4°). 
Our investigations in regard to the relations of the heart to the different vessels 
which pass to and from it are quite defective, and, like other points in the 
development of the organs, must be left for future research. 
The Bloodvessels. All the principal bloodvessels originate, like the heart, as 
channels, hollowed in the superior thickness of the subsidiary layer. We have 
already (p. 545 and 594; Pl. 9d, fig. 1, h, j?; Pl. 9e, fig. 5, y?) pointed to the 
fact, that the subsidiary layer becomes more or less separated from the layers 
above it; but, with the exception of the tracks along which vessels are formed, 
this gap is afterwards filled up again. After the initiatory steps are taken to form 
the heart, the anterior pair of the branchial aorte (PI. 12, fig. 7, 7) are developed : 
they appear as channels running obliquely forward and upward, one on each side 
of the head, from a common point of origin, the heart, (4,) toward the dorsal 
region (p. 547). At the same time, the branches (fig. 7, 7 7) of the afferent 
vessel, the omphalo-meseraic vein, commence as nearly transverse channels open- 
ing into the posterior end of the heart (4). These transverse channels (7, 7) 
are exactly in that bend of the subsidiary layer which forms the anterior edge 
of the abdominal cavity. Exterior to the body, the omphalo-meseraic vessel first 
appears as a broad and thick band, (7', 7',) which subsequently becomes hollow, 
and constitutes the vena terminalis. 
At the time the branchial fissures (Pl. 12, fig. 8, m, fig. 9, m) are formed, 
the three other branchial aorte (PI. 18a, fig. 11, 7, and note 2, p. 551) are 
developed between, and run parallel with them. <A_ little later, the branchial 
aorte empty into the two branches of the dorsal artery (Pl. 18a, fig. 11, j*, also 
fic. 7, 7). Beyond this we have nothing more to say in regard to the branchial 
aorte themselves besides what has already been stated in the section on the 
general development of the embryo. 
The aorta dorsalis, or descendens, originates as a forked vessel, one limb of 
the fork running along the right side, (PI. 18a, fig. 11, 7, y*,) and one along 
the left, (Pl. 18, fig. 7,) of the dorsal region, just behind the head, and unit- 
ing into one median channel (Pl. 9e, fig. 7, y?; Pl. 18, fig. 7) at a short dis- 
tance behind the heart. From this point it runs singly along the axis of the 
body to its posterior extremity; and finally, when the tail is developed, to the end 
of that organ (Pl. 13, fig. 2, w-e. 1, y*, p. 553). The anterior ends of the two 
branches of the fork of this vessel run forward into the head, (Pl. 18a, fig. 11, 
Jj») and eventually branch there very extensively (Pl 13, fig. 2). Soon after the 
forming of the aorta, lateral branches (Pl. 18, fig. 5, 7) run out from its right 
and left sides and pass into the vascular area, and its posterior end forks, and 
9 
joins the two lateral abdominal veins. Presently the allantoidian arteries (Pl. 13, 
J y 
