EARLIEST BLOOD-VESSELS IN MAN 457 



connection is shown in the drawing, the others are continued in 

 adjoining sections. We are deahng here, undoubtedly, with 

 endothelium, and the processes are typical angioblast cords. 

 The cords run usually, if not always, in well defined spaces in the 

 mesenchyma. 



The mesothelial cords, as well as the processes of the surrounding 

 mesenchymal cells, are left, as it were, attached to the outer wall 

 of the cavity, while the endothelium lies free within. Such 

 mesothelial cords are found in much older embryos, as for instance 

 in that described by Dandy (of seven segments) and in one of 

 about the same age kindly lent me by Prof. R. Meyer of Berlin. 

 They lead from the outer walls of the now well defined umbilical 

 vessels to funnel-shaped irregularities of the coelom wall, where 

 they are continuous with the surface mesothelium, frequently anas- 

 tomosing with each other. Since they seem involved in the process 

 of haemopoesis, I shall mention them again later. 



In addition to the method just described, whereby endothelium 

 is derived by delamination from the mesothelium of isolated 

 portions of the coelom, a more direct method seems to be shown 

 in the preparations studied. It will be remembered that in the 

 reconstructions of the Minot enibryo (and the same is true of those 

 of the Grosser embryo) the angioblast cords of the net could be 

 frequently traced to ingrowths from the coelomic mesothelium 

 without the intervention of the unlined spaces. Isolated portions 

 of such cords, in advance of the net in the Minot embryo (f g. 1) 

 are also thus connected, ^gain, in the Herzog enibryo, one of the 

 funnel-shaped ingrowths is continuous, as seen by reconstruction, 

 with a short cord (f g. 4, b) which it is impossible to follow far, but 

 which is similar to those forming the net in the chorion. These 

 cords are characterized by rather darkly staining protoplasm, and 

 by the absence of protoplasmic processes connecting them with 

 the surrounding mesenchyma, from which they are usually slightly 

 separated, leaving an extra-intimal space (fg. 5). Their resem- 

 blance to the endothelial cords which form the links between 

 angiocysts in all the vascular nets studied makes me believe that 

 they also are true endothelium, and that thus endothelium may 



