GROWTH OF BLOOD-VESSELS IN FROG LARVAE 45 



develop, but in which vessels, including the border vein and some 

 others differentiated in situations corresponding with the normal. 

 That growth of capillaries and larger vessels in embryos is regu- 

 lated not entirely by the metabolism of the tissues, but in part 

 at least by hereditary influences, is shown, he believes, by the 

 richness of the capillary plexus in the lung and the relatively 

 great size oJf the pulmonary arteries and veins, which, ' according to 

 Wiener,' are, before birth, four to six times as large as the weight 

 of the lung tissue justifies, in comparison with other organs. 

 Wiener studied the proportion between size of artery and weight 

 of organ. 



Support is lent to this view by the results of studies made on 

 embryos whose heart-beat has been eliminated experimentally 

 either by mechanical removal or by chemical inhibition. Dareste 

 (77) J. Loeb ('93), Patterson ('09), Knower ('07) and Stockard 

 ('15 A) agree in finding certain typical arteries and veins 

 formed in such embryos, in which the mechanical action of the 

 circulation has been eliminated — in fish, frog and chick embryos. 



The indications are that the truth lies between the two ex- 

 treme views; that what we are forced to call hereditary factors 

 do play a part, not only in the primary differentiation of blood- 

 vascular endothelium and its capacity for gro^vth by sprouting, 

 but in the formation of some of the main vessels in the embryo 

 (how great a part and how long exerted in embryonic life, has 

 not yet been cleared up, cf. Miss Sabin, '17, previously referred 

 to) that, on the other hand, the vascular system does become, at 

 an early stage, dependent, at least to a very great extent, upon 

 the regulative action of mechanical and chemical forces. 



Were it found that arteries and veins in latter stages are com- 

 pletely regulated as regards diameter, length, thickness of wall, 

 and position by the action of mechanical and chemical factors, 

 it would be quite compatible with our knowledge of the develop- 

 ment of other tissues and organs, to find that a crude pattern of 

 such mechanically controlled structures should reappear in the 

 embryo (Thoma, '93, p. 28). 



As to the precise nature of the mechanical and chemical fac- 

 tors which regulate the growth of the vascular endothelium, 



