344 PROFESSORS ARTHUR ROBINSON AND A. GIBSON. 



dorsal end of the mandibular arch by the union of a number of radicles. From 

 it- commencement, it runs, first caudally and then ventrally, to its termination in the 

 duct of Cuvier of the same side. In the cranial part of its course it lies dorsal to the 

 dorsal root of the first aortic arch and the most cranial part of the dorsal aorta, 

 ventral to the otic vesicle, and medial to the dorsal extensions of the first and second 

 branchial pouches. As it reaches the thorax it turns ventrally and crosses the 

 lateral side of the dorsal aorta a short distance caudal to the fourth branchial 

 pouch (fig. 58, PI. XIX). 



The Posterior Cardinal Veins.- — -Each posterior cardinal' vein commences in the 

 mesoderm near the caudal end of the embryo, and, as it runs towards the cranial end 

 of the body, it lies at first ventral to the corresponding Wolffian duct, then dorso- 

 lateral to the duct, and next on the ventro-lateral aspect of the more cranially situated 

 portion of the Wolffian body. After leaving the Wolffian body it runs along the 

 dorso-lateral wall of the pleuro-pericardial canal (fig. 58, PI. XIX), and, shortly 

 before its termination in the duct of Cuvier, it lies dorsal to the more caudal part 

 of the sinus v.enosus (fig. 58, PI. XIX). 



Ducts of Cuvier. — Each duct of Cuvier is formed by the union of the anterior 

 and posterior cardinal veins of the same side opposite the cranial end of the dorsal 

 wall of the sinus venosus, and a short distance caudal to the fourth branchial pouch 

 (fig. 58, PI. XIX). It is quite a short vessel, which is compressed from side to 

 side, and which runs ventrally and somewhat caudally round the lateral wall of the 

 pleuro-pericardial canal, to its termination in the sinus venosus (text-figs. 25 and 26, 

 pp. 341, 342). 



The Vitelline Veins. — The right and left vitelline veins are the largest blood- 

 vessels of the embryo, and each terminates in the corresponding ventro-lateral angle 

 of the caudal portion of the sinus venosus. The orifice of termination of each vein 

 is quite distinct from that of its fellow of the opposite side. 



The Umbilical Veins. — Each umbilical vein takes origin from the transverse sinus 

 which lies in the allantoic mesoderm at the caudal end of the body of the embryo, 

 and it receives a number of additional tributaries from the capillary plexus in the 

 allantoic mesoderm (figs. 59, 60, 61, PI. XIX). As it passes towards the heart each 

 vein lies in the most lateral part of the body wall of the corresponding side, and it 

 terminates, in the dorsal wall of the caudal part of the sinus venosus, a little dorsal 

 and medial to the termination of the vitelline vein (text-figs. 25, 26, pp. 341, 342, 

 and fig. 58, PI. XIX). 



The Coelom.— -The greater part of the intra-embryonic portion of the coelom is a 

 narrow cleft, which communicates freely with the extra-embryonic coelom at the mar- 

 gins of the embryo and at its caudal end. The more cranially situated portion of the 

 coelom is enclosed, and it forms a pericardial cavity and two pleuro-pericardial canals. 



The pericardium surrounds the heart. Its cranial extremity is situated at the 

 level of the second branchial arch. The caudal end lies at the cranial border of the 



