142 DEVELOPMENT OF ARTERIES IN FORELIMB OF PIG. 



with a rudimentary interosseous, a feeble ulnar, and a varying larger radial. First 

 in the primates appear the radial and ulnar as large and constant arteries. 



This dominant view received its first serious challenge at the hands of Miiller 

 (1903, 1904); its death blow was dealt by H. M. Evans. In the concluding para- 

 graph of his work on the morphology of the vascular system, Miiller states that 

 through his investigations on the comparative anatomy of the forelimb arteries 

 he finds that arterial tubes are derived from definite vascular nets; that the par- 

 ticular arterial arrangement in the various mammals does not permit them to be 

 arranged in any series from lower to higher forms; that it can not be established 

 that the ancestral form of the arm artery is an axial stem out of which the other 

 stems arise as branches of secondary or tertiary value. His findings show that a 

 general complicated network, such as he has described in the human embryo, forms 

 the primordium out of which particular branches arise. Mechanical influences, 

 working during ontogeny, are the determining factors of the various forms which 

 the arteries in the mammals assume. 



The present position is that Evans (1909) has reduced almost all vessels to a 

 primordial vascular net, extending it to the caudal aorta, the umbilical veins, etc. 

 Dr. Florence R. Sabin (1921) has participated in this, revealing how, in the chick 

 and pig, the angioblasts arrange themselves in diffuse or longitudinal form. 



Elze (1913) has opposed the view of Evans, his attack on the latter following 

 two lines. In escaping from the theory of predestination, Evans has based his 

 conclusions on the laws deduced by Thoma to explain the morphogenesis of blood- 

 vessels. Elze attempts to refute these laws by deducing from them the course and 

 form which the developing vessels should pursue and assume in deference to these 

 laws. It would not serve any useful purpose to analyze here examples which he 

 quotes to demonstrate the inapplicability of Thoma's postulates. Experimental 

 evidence would be necessary in order to determine the validity of these specula- 

 tive applications. The second line taken by Elze is to deny the universality of 

 the "net" theory. The specific exceptions he mentions, such as the aorta, cardinal 

 veins, and segmental arteries, have been the objects of particular study, and Evans's 

 paper on the aorta, cardinal and umbilical veins, and other blood-vessels indicates 

 that the strength of Elze's objections is not very great. Elze is not convinced of the 

 existence of the plexus arteriosus subclavius. It seems difficult to understand how 

 this objection can be maintained in the face of the investigations of Rabl (1906) 

 and Evans (1909) on the forearm of the bird, and those of Goppert (1910) on the 

 white mouse. Although I have not found the variability in the pig that Goppert 

 observed in the earliest blood supply to the forearm of the mouse, the present investi- 

 gation has clearly shown the polysegmental supply of the limb-bud and the plexi- 

 form arrangement of the early arm branches. The situation may therefore be 

 summed up by saying that the primordium of the vascular system lies in the vas- 

 cular net; that the vascular net depends upon the inherent properties of certain 

 cells to form blood-vessels and blood-cells, these properties being regulated by the 

 needs and activities of the surrounding tissues; that the circulation and vascular 

 pattern at any one time are adequate for the needs of the tissue and carry no impli- 



