THE DEVELOPMENT OF THE VASCULAR SYSTEM 



229 



the fore-gut. There are no observations on the origin of the heart in human 

 embryos, but it is reasonable to assume that it has the same double origin 



Dorsal aortic root 



Gut (pharynx) 



Pericardial 

 cavity (ccelom) 



Endocardium 

 (endothelium) 



Myocardium 



FIG. 204. Transverse section of a human embryo of 2.69 mm. von Spee, Kollmann's Atlas. 



Oral fossa 



Ventral aortic, 



trunk" " 



Ventricle 4 



Ant. cardinal vein 

 Duct of Cuvier 

 Umbilical vein 



Ventricle 



Atrium 



Diaphragm 



Duct of Cuvier 



Liver 



Duct of liver 



FIG. 205. Ventral view of reconstruction of human embryo of 2.15 mm. His. 



The ventral body wall has been removed. The vessels (in black) at the sides of the duct 



of the liver are the omphalomesenteric veins. 



as in other Mammals, although in embryos of 2 to 3 mm. the organ has 

 already become a single tube (Figs. 204 and 205). At this stage the tube is 

 somewhat coiled. 



