464 JOHN LEWIS BREMER 



of such different types as chick and rabbit or man may show dif- 

 ferences in details of the vascular development. 



Once given an endothelial net in the general area of the future 

 vessels, mechanical forces would, as pointed out in the paper so 

 often referred to, locate the portions of that net which are to 

 remain and form aorta, heart, etc., and the portions which are 

 to disappear because unfavorably placed. It is probable that all 

 the variations in size and shape of heart and aorta, and even in 

 position of the more lateral vessels, which are not infrequently 

 shown in the drawings of the operated embryos, are due to changes 

 in the normal tension of the germ layers, consequent on the injury. 

 That these changes must be great is shown in gross by the bending 

 of the whole body of such embryos away from the injured side. 

 A study of these embryos from this point of view would lead to 

 interesting results in the field of developmental mechanics. 



CONCLUSIONS 



In human embryos the earliest blood-vessels arise separately 

 in the yolk-sac and in the body-stalk, by multiple anlages. 



The anlages in the body-stalk (and perhaps also in the yolk-sac 

 (cf. Jung's figure 17) are funnel-shaped ingrowths of the surface 

 mesothelium, which is present as a definite layer only on the two 

 areas mentioned. By partial fusion of the walls of an ingrowth a 

 portion of the coelom, still bordered by mesothelium, may be cut 

 off as a separate cavity, lying deep within the substance of the 

 body-stalk. 



The endothelium seems to arise either (a) by delamination from 

 the walls of such a detached portion of the coelom, or (b) by direct 

 extension, in the form of an angioblast cord, from the mesothelial 

 ingrowth. From the endothelium, by whichever method devel- 

 oped, further extension is by means of the angioblast cords, which 

 grow apparently through the surrounding mesoderm. 



True blood-islands may occasionally arise by the multiplication 

 of the cells of the mesothelial ingrowths, or scattered blood cor- 

 puscles may arise singly within these ingrowths. 



