HEAD-FOLD TO TWELVE SOMITES 121 



(1) that it is an ingrowth of the extra-embryonic vessels, (2) that it arises 

 from the mesoblast in 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 aorta3 are much better developed than the posterior por- 

 tion; indeed, in places there appears to be a complete hiatus 

 between cephalic and trunk aortse. The impression gained is that 

 a large part of the cephalic aortaB 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 aortae (Fig. 55), and an arterial pathway is 

 thus established from the heart by way of the ventral and dorsal 

 aortse to the vascular network of the splanchnopleure. 



The arterial system consists at thirty-three hours (12 s stage) 

 of the following parts: (1) ventral aorta; (2) first visceral or 



