294 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



and liver (fig. 314). 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 omphalo- 

 mesenteric (omphalomesaraic) veins (fig. 314). 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 omphalomesenterics 

 (fig. 313). These transverse vessels continue laterally between the 



Fig. 313. Fig. 314. 



Fig. 313. — Diagram of the formation of the heart tube, showing the descending 

 mesothelial plates from above, c, coelom; cd, first appearance of the Cuvierian ducts; 

 h, grooves to form heart and ventral aorta; /, liver; m, mouth; ma, mandibular artery; 

 om, omphalomesenteric veins; so, sp, somatic and splanchnic walls of coelom. 



Fig. 314. — Early stage of the heart; the descending plates of fig. 313 have met, 

 forming the heart and ventral aorta, c, peritoneal coelom; p, pericardial coelom; ppc, 

 pericardio-peritoneal canals; other letters as in fig. 313. 



lateral plate and the ectoderm, forming the venous trunks known 

 as the ducts of Cuvier (tnmci transversi), the other relations of 

 which will be described later (p. 310). The ccelom on either side of 

 the heart is restricted behind by the ridge formed by the Cuvierian 

 ducts (fig. 314); with growth this interruption grows larger, the 

 result being a transverse partition, the septimi transversimi (fig. 

 11), which bounds the pericardial cavity behind and separates it 

 from the rest of the coelom, the peritoneal cavity. At first this sep- 

 tum is incomplete, and in the myxinoids and elasmobranchs it never 

 closes dorsally to the omphalomesenterics, but leaves two openings. 



