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TEXT-BOOK OF EMBRYOLOGY. 



endothelium and destined to become the endocardium, the outer wall of a 

 thicker mesothelial layer and destined to become the myocardium the two walls 

 separated by a considerable space. The organ hangs, as it were, in the primi- 

 tive pericardial cavity (ccelom), connected at its cephalic end with the ventral 

 aortic trunk and at its caudal end with the omphalomesenteric veins. 



In all Mammals thus far studied the principle of development in the earlier 

 stages is essentially the same as in the chick. The double origin of the heart is 

 even more marked because of the relatively late closure of the fore-gut. There 

 are no observations on the origin of the heart in human embryos, but it is 



Aorta 



Gut (pharynx) 



cavity (coelom) 



FIG. 195. Transverse section of a human embryo of 2.69 mm. von Spec, Kollmann's Atlas. 



reasonable to assume that it has the same double origin as in other Mammals, 

 although in embryos of 2 to 3 mm. the organ has already become a single tube 

 (Figs. 195 and 196). At this stage the tube is somewhat coiled. 



The origin of the endothelium of the heart (endocardium) is not known with 

 certainty. Some investigators in their researches among the lower Vertebrates 

 have suggested that it is derived primarily from the entoderm ; others have sug- 

 gested a possible derivation from both entoderm and mesoderm; still others hold 

 that it is derived from the mesenchyme (mesoderm) . It seems to be undis- 

 puted, however, that the muscular wall of the heart (myocardium) is a derivative 

 of the mesothelium (mesoderm). 



