318 Herbert M. Evans. 



which tend everywhere to form a mesh- work or plexus; arteries and 

 veins are always subsequent formations from snch capillary plexuses 

 due to the transforming influence of the circulation. 



The former conception would appear to be held by most of the 

 workers in angiogenesis, though most of the descriptions of the devel- 

 opment of vessels are so worded as to avoid a lucid statement on 

 this fundamental point. I may refer, for instance, to the many 

 admirable researches of Hochstetter, where, though many important 

 facts concerning the chief embryonic vessels are clearly given, one 

 may look in vain for anything bearing on this point. In the case 

 of the limb vessels, for instance, we must imagine from his descrip- 

 tion that the single axial vessel grew out into the core of the limb. 

 Very recently Curt Elze^^ has ranged himself with those who would 

 recognize such a j)rocess as the means of development of all the 

 body's vessels and as definitely opposed to the idea of a capillary 

 plexus anlage for any of them. On the other hand, Hans Rabl 

 and Erik Miiller have supported vigorously the latter idea, the 

 foundation for which had been laid in the great paper of Thoma 

 on the origin of the chick's yolk vessels. 



There is plenty of morphological evidence in the adult body for 

 a preexisting plexus or net-like condition of all the vascular trunks. 

 The remarkable number of variations in the position, system of 

 branching and anastomoses cannot be explained as satisfactorily on 

 any other basis. Thus, without knowledge of conditions in the em- 

 bryo, Aeby^^ and Baader^^ promulgated such a plexus origin for 

 blood-vessels many years ago. 



Professor His did not hesitate to state that the main vessels in 

 the embryo were derived from net-like anlagen, but it remained for 

 Thoma^*^ to make the meaning of all this very significant. 



Thoma observed that in its early stages the system of the vitelline 

 vessels in the chick formed a strikingly uniform simple plexus of 



"Elze. "Beschreibung eiues meuscblicbeii Embryo von zirka 7 mm. griisster 

 Lilnge." Auat. Hefte, 1-35, 1907. 



"Aeby. "Der Bau des meuscblicbeii Korpers." 1871. 



"Baader. "Ueber die Varietiiten der Armarterieu des Menscben." Inaug. 

 Diss. Beru, 1866. 



""Tboma, E. "Untevsiicbungeu iiber die Histogenese nnd Histomecbaiiik 

 des Gefilsssystems." 1893. 



