RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 23 



(4) Confluence of these endothelial-lined spaces to form 

 larger and larger vascular areas of intercommunicating channels, 

 containing a clear plasmatic fluid, which circulates in the chan- 

 nels in response to the establishment of cardiac pulsation. 



(5) Addition to the plasmatic contents of these channels, 

 coincident with their further growth and extension into a con- 

 tinuous circulatory system, of cellular elements, derived from 

 the mesoderm and specially modified to acquire haemoglobin 

 and function as red blood cells. These cells, by solution of 

 tissue continuity, are liberated from the blood islands which are 

 first surrounded by the confluent spaces of the capillary anlages. 



(6) Subsequent differentiation of the adventitia, with speciali- 

 zation of districts by cardiac concentration, amalgamation of 

 the plexus into larger arterial and venous channels, definition 

 of permanent capillary areas, valve and septal formation, etc. 



The striking features of this ontogenetic history of the blood 

 vascular system are: 



1. The relatively late addition to the preformed non-cellular 

 circulation of free cell elements, which, as the red blood cells, 

 stamp, from the period of their liberation and inclusion in the 

 circulating plasma, the resulting vascular system as haemal. 



2. The common origin from mesoderm of both characteristic 

 components of the haemal system, viz., the vascular endothelium 

 and the red blood cell. Both the vascular intima and the free 

 cell contents of the channels lined by this intima appear as highly 

 modified derivatives of the same mesodermal cellular ancestors, 

 which constitute the cell-strands of the earliest period of vascular 

 development. In their first inception the systemic lymphatic 

 vessels of the mammalian embryo, as distinguished from the 

 jugular lymph sacs of venous origin, repeat in every detail the 

 primary stages of the developing haemal capillaries, prior to the 

 inclusion within the lumen of the latter, of the cellular contents 

 of the blood islands. They can be identified as distinct struc- 

 tures as soon as the blood channels proper have differentiated. 

 Before that period direct observation cannot determine, in case 

 of individual mesenchymal spaces, whether they are eventually 

 to become part of the lymphatic or of the blood vascular system. 



