44 ELIOT R. CLARK 



(better 'hemangiotactic') substances outside the capillaries. Ac- 

 cording to Marchand ('01, p. 148), Leber ('88) first suggested 

 this explanation, to which Marchand is slightly inclined. It 

 was suggested again by J. Loeb ('93) as an explanation for the 

 growth of vessels in fish embryos whose heart action was elimi- 

 nated by the action of chemical substances. Evans makes a 

 similar suggestion. In each case it has been proposed merely 

 as a tentative hypothesis and has not been tested. 



Over against this group of investigators whose studies have 

 gone to show that blood-vessels are regulated in their growth by 

 the action of mechanical and chemical factors, and some of whom 

 have attempted to define this regulation in terms of specific 

 laws of growth, there are others who have supported the view 

 that mechanical factors play little if any part in determining the 

 formation of arteries and veins, and who attribute it rather to 

 the action of hereditary influences. Possibly the strongest ad- 

 herent of this view is Hochstetter, who has made so many im- 

 portant studies on the comparative anatomy of the vascular 

 system. His view is probably most concisely presented by his 

 pupil, Elze ('12) in an article criticizing the conclusions of Evans 

 and Thoma. In brief, it is that the primitive form of the" vascu- 

 lar system is not a capillary plexus, but a single artery and vein, 

 such as is formed in the limbs and digits of amphibians, and also 

 in the segmental arteries ; while capillary plexuses are secondary 

 formations. 



Now it is interesting that support for this view has come in 

 part from the two men who have been most prominent in advocat- 

 ing the regulating action of mechanical factors, namely, Thoma 

 and Roux. Thoma ('93, p. 28) mentions that the aorta is de- 

 veloped as a definite vessel before the heart commences to beat, 

 while Roux emphasizes a first stage in the development of the 

 vascular system, as of other systems, in which differentiation 

 and growth take place as a result of heredity (preformation) — 

 a stage which includes the formation of ' the anlage of the typi- 

 cally laid down chief vascular stems' ('95, pp. 326-7, footnote). 

 Roux bases this conclusion on chance observations made on the 

 area vasculosa of chick embryos, in which the embryo failed to 



