876 THE VASCULAR SYSTEM. 
reversed, and at this period the pericardial cavity which, like the area, is semilunar in 
shape, extends from side to side beneath the foregut, and its cornua are continuous at the 
sides of the foregut with the general body cavity of the embryo. Subsequently this con- — 
tinuity is obliterated, and the pericardial cavity is separated from the pleuro-peritoneal — 
part of the general body cavity or coelom. | 
The mesoderm at the posterior end of the reversed pericardial region, where the 
somatic and splanchnic layers are continuous, 7.e. Just in front of the umbilicus, increases 
in thickness and forms a semilunar mass, the septum transversum, in which the liver and 
the ventral part of the diaphragm are formed. The latter extends mesially from the 
anterior wall of the body to the foregut, immediately in front of the gastric dilatation, whilst 
laterally it forms two faleiform projections which encroach from without upon the portions 
of the ccelom which lie at the sides of the foregut. Ultimately the lateral portions of the 
diaphragm pass inwards, and, fusing with the mesentery of the foregut, they separate the 
pleuro-peritoneal portion of the ccelom into three par ts—two anterior, one on each side of 
the foregut, the pleural sacs, and a posterior, the peritoneal cavity. 
When the pericardial region is completely reversed the two vessels developed in 
its splanchnic layer lie side by side in what is now its dorsal wall. Posteriorly they are 
continued into the lateral part of the septum transversum, and through it they pass to 
the wall of the yolk sae. Anteriorly they are 
Ist cephalic aorticarch @ontinued, as the first cephalic aortic arches, 
Anterior primitive through the mandibular arches which have 
ventral aorta Sue? & i 
developed at the sides of the bucco-pharyngeal 
Primitive dorsal aorta membrane, to the dorsal wall of the gut, where 
they pass backwards beneath the paraxial meso- 
‘Vitelline vein derm and provertebral somites to the posterior 
end of the body, whence they are continued, 
! Umbilical vein in the human subject, along the body stalk to 
i } the placental portion of the chorion, giving off 
1 og Sl | branches to the walls of the alimentary canal 
} \ [Pb Branches to yolk sae and yolk sac. 
—— a; / These two primitive embryonic vessels are 
N WA \ A the primitive aorte. After the formation of 
the cephalic and caudal folds, each primitive 
Vitelline artery ee 
aorta may be looked upon as consisting of three 
Posterior primitive. 
——~entral aorta parts united by two arches :—an anterior ventral 
—Primary caudal arch part, the anterior ventral aorta, situated partly 
Allantoic artery in the septum transversum and partly in the 
J dorsal wall of the pericardium and the root of 
ATA ie) eee the neck ; a dorsal part, the primitive dorsal 
NBN aorta, which extends beneath the paraxial meso- 
iTily RON? TIAGERIGIE nL TE Cet NVERCE derm from the dorsal end of the mandibular 
CULAR SYSTEM BEFORE THE Formation or arch to the tail fold; a posterior ventral part, 
THE HEART. the posterior ventral aorta, which passes to 
the yolk sac; the continuation of the posterior 
ventral aorta to the yolk sac soon atrophies, however, and the vessel is then prolonged — 
from the posterior part of the ventral wall of the body to the placenta by a new branch. 
The two arches which unite the three main portions of each primitive aorta together are 
an anterior, the first cephalic aortic arch, which lies in the mandibular arch and passes 
from the anterior ventral aorta to the anterior end of the primitive dorsal aorta, and a 
posterior, the primary caudal aortic arch, which passes in the tail fold and at the side 
of the hind gut, from the primitive dorsal aorta to the posterior portion of the primitive 
ventral aorta. 
As development proceeds a series of transformations occurs in the various sections of 
the primitive aortee. These transformations are, with few exceptions, alike on the two sides, 
but the transformations which occur in one section are entirely different from those met 
with in the other sections ; therefore each part must, to a certain extent, be considered 
separately. 
Each anterior ventral aorta is divisible into three parts. The posterior part lies in the 
septum transversum. Posteriorly it forms the terminal portion of the vitelline vein, and 
carries the blood from the wall of the yolk sac. For a long time it remains separate from 
its fellow of the opposite side, but afterwards the two vitelline veins unite to form a 
common stem, which terminates at first in the posterior part of the heart, and subsequently 
in the liver. The anterior section of the posterior part of the anterior ventral aorta 
' 
