200 THE DEVELOPMENT OF THE CHICK 



appears strikingly bilateral at this time. (2 and 3) The ducts of 

 Ciivier are made up by the union of all the somatic veins. Each 

 is formed primarily by the union of the anterior and posterior 

 cardinal veins. The anterior cardinal vein receives all the blood 

 of the head, and thus includes the first three segmental veins. 

 It also receives at its point of junction with the posterior cardinal 

 vein a branch from the floor of the pharynx, the external jugular 

 vein. The posterior cardinal vein receives (1) all the segmental 

 veins of the trunk, of which there are twenty-nine pairs, running 

 in the intersomitic septa between the fourth and thirty-third 

 somites, and the veins of the Wolffian body of which there are 

 several to each somite concerned, as described in the account 

 of that organ. 



The development of the vascular system up to the stage just 

 described will now be taken up. 



Developmeiit of the Heart, (a) Changes in the External Form. 

 In the last chapter we traced the origin of the heart up to the 

 time when it is a practically straight, undivided, somewhat 

 spindle-shaped tube lying below the floor of the pharynx, to which 

 it is attached by its dorsal mesentery (mesocardium). Posteriorly 

 its cavity divides into the omphalomesenteric veins which run 

 in the side-walls of the anterior intestinal portal. The heart is 

 lengthened backwards by the concrescence of the omphalo- 

 mesenteric veins and the most posterior division of the heart 

 (the sinus venosus) is established in this way between the stages 

 of 12 and 18 somites; it is marked by a broad fusion with the 

 somatopleure (mesocardia lateralia) through which the ducts of 

 Cuvier enter the heart. 



At the stage of sixteen somites the duct of Cuvier lies opposite 

 the hind end of the second somite on the right side, and a little farther 

 back on the left side; and the somato-cardiac fusion (mesocardium 

 laterale) in which it lies is of the width of about one and a half somites. 

 On the right side the duct of Cuvier lies a little in front of, and on the 

 left side a little behind, the point of union of the omphalomesenteric 

 veins; thus the posterior end of the heart is not fully formed at the 

 stage of 16 s, but is at the stage of 18 s. The subsequent fusion of the 

 omphalomesenteric veins produces the so-called ductus venosus, or 

 main splanchnic vein, which is therefore a posterior continuation of the 

 sinus venosus. 



The cardiac tube proper lies between the origin of the aortic 



