ORIGIN OF VASCULAR TISSUES 43 



mesenchymal process, though it is often less than that of the cord 

 nuclei. The cords tend to form nets by anastomosis of larger mesh 

 than the mesenchymal net, and angio cysts wherever space is given. 

 They are usualty sharply defined from the surrounding tissue, and 

 may show an extraintimal space. 



In this sense the angioblast is not necessarily the unit vascular 

 anlage which grows into the embryo from the yolk-sac. Its 

 only prerequisite is that it shall grow continuously (from one or 

 many sources) through the embryonic tissue as 'solid cords of 

 cells.' 



It is to be hoped that the term 'angioblast' may never be appHed to 

 any structure except to this theoretically early differentiated, preco- 

 ciousty segregated, yolk-sac, unit vascular anlage which has been 

 claimed to give rise to all the vascular tissues. If no such exclusive 

 and sole 'unit anlage' exists, then the term needs and deserves to be 

 discarded. It has been used in the literature for such a great number 

 of years that its meaning is ver}^ definite and well established. The 

 term is so intimately related to the concepts of specificity and ingrowth 

 from the yolk-sac of all vascular tissues that it can never, with pro- 

 priety, be attached to any embryonic structure except the early yolk- 

 sac vessels. It is to be hoped that no observer will ever find it neces- 

 sary to designate individual mesenchyme cells as 'angioblasts,' even 

 provided he should become convinced that such cells are vasofactive. 

 The term 'angioblast' implies a 'unit anlage;' it implies a collectively 

 multicellular tissue, and therefore can not properly be used in connec- 

 tion with a single cell. If there be such a thing as angioblast, there is 

 but one angioblast and it has but one method, place, and time of origin. 

 To no isolated cell should be attached a term which for decades has 

 been applied to a group of yolk-sac blood-vessels. Strictly speaking 

 it would be inconsistent to designate as 'angioblastic cords' any solid 

 forerunners of intraembryonic vascular tubes, even though the ingrowth 

 theory were correct. Angioblast can not properly be applied to solid 

 or tubular endothelium which has arisen by sprouting; the term implies 

 .the original anlage which does the sprouting, and which does not, 

 according to the angioblast theory, arise from preexisting vascular 

 tissue; it does not include the sprouts, or the sprouts of sprouts. 



Finally the view of Rabl might be mentioned. He is responsi- 

 ble for the dictum "Endothel stammt nur von Endothel." 

 Taken literally such a statement would relegate vascular endo- 

 thelium to a category in which the germ cells, and perhaps some 

 of the most primitive organisms are generally regarded as unique 

 because of their supposed immortality. 



