78 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



two separate endothelial tubes instead of a single angiocystic mass. One may con- 

 clude that in man the two halves of the heart fuse at a very early period in their 

 development, if there is actually a stage in which they are two such independent 

 structures. It is conceivable that in the heart before us we might later have found 

 two quite independent endothelial tubes, but the probability seems very remote. 

 In man fusion antedates the complete canalization of the heart, while in most forms 

 the reverse is the case (Parker, 1. c; Bremer, 1912; Schulte, 1916; Wang, 1918). 



As regards the actual processes which give rise to the lumen of the heart, the 

 same principles obtain in man as have been described in the cat by Schulte (1. c). 

 ( ompared with endothelium formation elsewhere, there is in the heart a relatively 

 large amount of mesenchyme and very little endothelium. We can find nothing 

 indicative of the origin of this mesenchyme or of its being increased in amount 

 except by its own proliferation. It is everywhere at a considerable distance from 

 the myoepicardial mantle, but its most caudal extensions lie very close to the ento- 

 derm. The entire heart has obviously arisen in loco and not by the invasion of 

 vasculogenic elements from without. It is not possible to say to what extent the 

 heart, as shown in figures 7 and 8, is to be derived from a single paired or even 

 unpaired primordium by a simple process of growth; there has doubtless been not a 

 little accretion of angioblastic material, and this probably more extensive at the 

 cephalic end than elsewhere. The continued formation of angiocysts, their exten- 

 sion and coalescence, will soon transform the impervious mass as it now exists into 

 an open passage between the omphalomesenteric veins and the first aortic arches. 

 By this time we might expect to find a free communication between the venous end 

 of the heart and the anterior part of the vitelline plexus and also a dorsal aorta 

 continuous and patent throughout its extent. 



Posterior to the first pocket the dorsal aorta? are in process of formation, as 

 evidenced by the presence of a number of small vesicles, or even less definitely 

 limited spaces, in the rather dense mesenchyme close to the roof of the fore-gut. 

 About opposite the intestinal portal the right aorta gradually becomes more evident 

 and can be traced backward, lying close to the entoderm, as far as or possibly a little 

 beyond the point where it has its lateral connection with the posterior portion of the 

 vitelline plexus, and thence with the artery of the body-stalk (fig. 4). Toward its 

 posterior end the aorta is distinctly dilated and there is here a possible second, more 

 anterior vitelline artery in process of formation. Anterior to this dilation is a small, 

 much constricted, but probably pervious segment of the vessel. From the intes- 

 tinal portal to the vitelline artery the aorta is for the most part a definitely lined 

 channel; the constriction just noted may possibly be solid, and also near the origin 

 of the vitelline artery its walls are, in a few places, almost deficient. Whether its 

 posterior end is open or really closed off is very difficult to determine. Beyond the 

 latter point a few scattered but conspicuous cells are found, quite different from 

 the other cells near them, especially as regards their larger, longer, and more deeply 

 staining processes. They are to be looked upon as material for the further growth 

 and extension of the aorta or its branches. Situated in the line of the aorta and 

 laterally toward the yolk-sac, these cells show the orientation described for similar 



