38 ELIOT R. CLARK 



extension and development of the system after the primary- 

 differentiation has taken place and after the circulation has been 

 established, and it is with this second stage that the studies 

 here reported are concerned. In this stage, which continues 

 throughout life, the vascular endothelium spreads through the 

 growing organism, arteries and veins develop, until the extensive 

 and complicated vascular system of the adult is perfected. It 

 is principally characterized by the formation of new vessels by 

 the sending out of sprouts from the vessels already present, in- 

 stead of by the transformation of mesenchyme, of other undifferen- 

 tiated cells, as in the first stage, and by the action on the vessels 

 of the mechanical and chemical factors concerned with the circu- 

 lation of blood and interchange of substances through the wall. 



In spite of the abundant evidence in favor of this mode of 

 spreading of the vascular endothelium, after its primary differ- 

 entiation, there ■ are observers who adhere to the view that at 

 any time throughout life, mesenchyme (or other undifferentiated 

 ceUs) may be transformed into vascular endothelium. This 

 view is held by Maxinow, Weidenreich, and Mollier (cf. dis- 

 cussion in Schulte, '14,) who believe that, not only may reticulum 

 cells and leucocytes be transformed into blood-vessel endothe- 

 lium, but that the reverse transformation may take place — in 

 brief, that vascular endotheUum is not a specific tissue, but is 

 interchangeable with the other tissues mentioned. The evidence 

 for this view has in no case been conclusive. It is clear, however, 

 that the manifestation of the property of sprouting does not 

 form a sharp boundary line in time of development between 

 stages, for apparently sprouting commences before the differen- 

 tiation of endothelium is everywhere complete (cf. Stockard 

 '15, B and Sabin '17). It is probable that the period of over- 

 lapping is very short. 



It is also clear, particularly from the studies of Miss Sabin 

 ('17), that the development of arteries and veins takes place to 

 some extent before the circulation is established. In chick em- 

 bryos she found that, before circulation starts, part of the aortae, 

 the two vitelline veins next the heart, parts of the cardinal veins 

 and the duct of Cuvier are clearly present as definite vessels. 



