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 does 

 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 



