ca 
CIRCULATORY ORGANS. 271 
the extent of which is decreased by the fusion laterally of the somatic 
and splanchnic walls (fig. 277). 
In front of and behind this tube the descending lateral plates are 
kept from meeting in the middle line by the projections for the mouth 
and liver (fig. 276). Vascular cells, however, are formed in these 
regions and these furnish the lining of tubes on either side, arising 
in the edges of the lateral plates. These tubes consequently diverge 
from the myoepicardium in front and behind and form the first stages of 
the vessels connected with the heart, the anterior pair giving rise to the 
mandibular arteries, the posterior to the omphalomesenteric veins. 
At about the same time a transverse tube appears on either side, which 
connects with the heart tube, just in front of the division into omphalo- 
mesenterics (fig. 276). These transverse vessels continue laterally 
between the lateral plate and the ectoderm, forming the venous trunks 
known as the ducts of Cuvier (trunci transversi), the other rela- 
tions of which will be described later. The ccelom on either side of 
the heart is restricted behind by the ridge formed by the Cuvierian 
ducts (fig. 277); with growth this interruption grows larger, the result 
being a transverse partition, the septum transversum, which bounds 
the pericardial cavity behind and separates it from the rest of the ccelom, 
the peritoneal cavity. At first this septum is incomplete, and in the 
elasmobranchs it never closes dorsally to the omphalomesenterics, but 
leaves two openings, the pericardio-peritoneal canals (fig. 277). 
Elsewhere the pericardial and peritoneal cavities are entirely separate 
in the adult. 
In teleosts and amniotes, where the early embryo is closely appressed to the 
very large yolk sac, the development of the heart is modified. At first the pharynx 
is not complete below but communicates ventrally with the yolk. Hence the two 
hypomeres are prevented, for a time, from meeting ventrally. Each, however, is 
accompanied by its vascular cells; its edge becomes grooved and the grooves are 
rolled into a pair of tubes, lined with endocardium, so that for a time the anlage of 
the heart consists of two vessels, each connected in front and behind with its own 
mandibular artery and omphalomesenteric vein, and is surrounded with its 
pericardial sac. Later the two tubes approach and fuse, with the formation of 
mesocardia as before: these latter soon disappearing, leaving the whole much as 
in the small yolked forms. 
In the early stages the pericardium is relatively large, but it doés 
not keep pace with the growth of the other parts, until finally in the adult 
it is only large enough to accommodate the changes in size and shape 
