200 THE DEVELOPMENT OF THE CHICK 
appears strikingly bilateral at this time. (2 and 3) The ducts of 
Cuvier 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. 
Development 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 conecrescence 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 16s, but is at the stage of 18s. 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 
