HEAD-FOLD TO TWELVE SOMITES 121 
(1) that it is an ingrowth of the extra-embryonic vessels, (2) that it arises 
from the mesoblast 7n situ, (3) that it arises from the entoderm in situ. 
Appearances such as that shown in Fig. 53 favor the last view. 
The heart is then a double-walled tube attached to the floor 
of the pharynx. The posterior end rests squarely against the an- 
terior intestinal portal and is continuous with the rudiments of 
the splanchnic veins running in the diverging folds of the portal; 
the anterior end of the heart is continued as a simple endothelial 
tube (ventral aorta) as far forward as the oral plate, where it is 
divided in two (Figs. 62, 64, etc.). 
This primitive simplicity of the cardiac tube continues through- 
out the period considered in this chapter without substantial 
alteration. The heart increases in length with considerable 
rapidity, but being attached at its anterior and posterior ends by 
the aortic and venous roots respectively, it is forced to bend, 
nearly always to the right, so that a convexity of the heart 
appears to the right of the embryonic head, at about the 11-12 s 
stage (Figs. 63, 64). About this time the mesocardium (dorsal 
mesentery of the heart) disappears except at the posterior end, 
and the cardiac tube thus becomes free except at its two ends. 
The Embryonic Blood-vessels. The dorsal aorta arises from 
the median edge of the vascular network, which extends across 
the pellucid area in the splanchnopleure. At the stage of 7-9 
somites, it has reached the nephrotomic level. The marginal 
meshes gradually straighten themselves out into a longitudinal 
vessel, continuous with the net-work at the sides and behind. 
Only the trunk part arises in this manner. The cephalic part 
arises by forward growth of the trunk part or from mesenchyme 
in situ. In some embryos the most anterior portions of the 
cephalic aorte are much better developed than the posterior por- 
tion; indeed, in places there appears to be a complete hiatus 
between cephalic and trunk aorte. The impression gained is that 
a large part of the cephalic aorte arises in situ. Some series of 
sections are practically conclusive in this respect (8-9 somites). 
A connection is then formed around the anterior end of the fore- 
gut with the ventral aorte (Fig. 55), and an arterial pathway is 
thus established from the heart by way of the ventral and dorsal 
aorte to the vascular network of the splanchnopleure. 
The arterial system consists at thirty-three hours (12s stage) 
of the following parts: (1) ventral aorta; (2) first visceral or 
