44 FRANKLIN P. REAGAN 



II. THE LOCAL ORIGIN THEORY 



The view opposed to the angioblast theory is that of local 

 origin. According to this view, mesenchyme may, in practically 

 any region of the body, transform into vascular tissue. The 

 cells which bomid an intraembryonic blood-vessel are not in 

 direct lineage with those which line the early vessels on the 

 yolk; they have not come into being as ingrowths from the 

 early yolk-sac vessels or 'angioblast;' they have not necessarily 

 come from preexisting endothelial cells, though some of them 

 may have had such an origin, inasmuch as local origin does not 

 preclude the possibility of growth during or following the process 

 of local vascular formation. Additions to endothelium already 

 formed may take place by (1) proliferation of cells already 

 formed, (2) addition of single mesenchyme cells, (3) addition of 

 solid cell aggregates, (4) addition of already formed endothelial 

 cavities the lining cells of which have differentiated locally, in 

 and from the mesenchyme , and (5) by the active migration and 

 alignment of single mesenchyme cells to form vascular cavities. 

 The local origin theory holds that blood-cells are not necessarily 

 descended from a primitive yolk-sac 'angioblast,' but that mesen- 

 chyme within the embryonic body is capable of giving rise to 

 blood cells. Advocates of the local origin theory do not believe 

 that the vascular anlagen are necessarily differentiated at a very 

 early stage of development, as claimed by His, or collectively 

 and at one time as stated by Minot; advocates of the mesen- 

 chymal theory recognize that there a^e certain regions in which 

 a precocious production of vascular tissues takes place, but they 

 claim that such regions are not the only regions in which such, 

 tissues are formed. Advocates of the local origin theory recog- 

 nize various intraembryonic regions in which there is a first- 

 hand production of vascular tissues, even relatively late in 

 ontogeny, and quite independent of such processes in the yolk- 

 sac. The angioblast theory regards endothelium as a tissue of 

 high specialization, quite foreign in nature to mesenchyme and 

 quite removed from it genetically. The local origin theory 



