RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 21 



develops de novo in the adult under appropriate normal con- 

 ditions. 



Furthermore, this endothelial characterization of modified 

 mesodermal cells is from the beginning a multiple process, start- 

 ing independently at innumerable separate and discrete points 

 of the vascular area, and becoming only subsequently continuous 

 by confluence of the individual separate anlages. This fact is 

 of importance in drawing general conclusions as to the later 

 extension of vascular endothelium, whether haemal or lymphatic. 



Up to this point the histogenetic and physical characters of 

 all developing vascular structures are identical. The picture 

 just described applies equally to the earliest definite anlages of 

 the haemal capillary system, and, as I shall show, to the first 

 appearance of the earliest lymphatic structures of the body. 

 In the case of the definite blood-vascular channels of the verte- 

 brate embryo, however, a further developmental change occurs, 

 namely, the addition of free, specially modified, mesodermal 

 cells, as the red blood cells, to the clear plasma circulating in the 

 channel-system of the earliest capillary anlages in response to 

 the first pulsations of the heart. 



The first blood vessels of the area pellucida appear, at least 

 in part, to develop independently of the so-called "Blood islands." 

 These latter, originally, form broad cellular strands composed of 

 closely packed uncolored cells, which are only distinguished from 

 the solid strands of the earliest vascular anlages of the area pellu- 

 cida by greater size and massiveness. After the vascular cell- 

 strands of the area pellucida have developed in their interior the 

 intercellular closed oval or round spaces of the first capillary anlages, 

 similar spaces also appear in the more massive cell strands of the 

 peripheral portion of the area vasculosa. In the subsequent 

 confluence of these discretely developed spaces to form the early 

 capillary reticulum, the blood islands become more and more 

 surrounded by the forming channels and are thus separated from 

 the adjacent tissues. New endothelial-lined spaces continue to 

 develop on the surface of the blood-islands, enlarge and join the 

 system of connected channels. The cells forming the walls of 

 these primitive capillaries become, as above stated, transformed 



