GROWTH OF BLOOD-VESSELS IN FROG LARVAE 57 



out from the arterial rather than the venous end of the capillary, 

 since, obviously, the pressure is higher in the arterial end. This, 

 however, is not the case — at least, in the tad-pole's tail new 

 sprouts grow out as frequentl}^ from the venous as from the ar- 

 terial ends. Loeb ('93) suggests that the explanation of the new 

 growth of blood-capillaries must l^e sought in the stimulus ex- 

 erted by specific chemical substances outside the capillary. A 

 similar suggestion is made by Evans ('09 B, note, p. 296), who 

 says in discussing vascular and non- vascular areas in embryos : 

 ''^Ve have to do here, perhaps, with a matter of cell chemistry 

 or tropisins, for endo.thelium apparently avoids certain areas in 

 the embryo — the non-vascular areas." In discussing this paper 

 (see also '12, p. 584), Thoma ('11) argues that the findings of 

 Evans fit in wdth his hypothesis, explaining nonvascular areas in 

 the embryo as areas in which the pressure is high, due to the 

 compactness of the tissues in such areas. As a result of this, 

 according to Thoma, the difference in pressure betw^een the blood 

 inside the capillary and the fluid outside is less in such areas than 

 in looser tissues where he supposes the pressure to be lower. 

 There appears, however, to be a valid objection to the sugges- 

 tion of Loeb and of Evans, which is found in the variations in 

 the richness of the capillary plexus in the different organs and 

 tissues of the adult. This is summarized as follows in Kolli- 

 ker's Gewebelehre ('02, p. 670) : 



Bestimmend ftir die Anordnung der Kapillaren ist die physiologische 

 Leistung, und ergiebt sich als allgemeines Gesetz, dass, je grosser die 

 Thatigkeit eines Organes, beziehe sie sich nun auf Bewegung oder 

 Empfindung, auf Ausscheidung oder Aufsaugung, vor allem in den 

 Lungen, der Schilddriise, der Leber, den Nieren, dann in den Hiiiiten 

 und den Schleimhaiiten, viel weiter in den Organen, die nur behufs 

 ihrer Ernahrung imd zu keinen anderen Zwecken Blut erhalten, wie*in 

 den Muskeln, Nerven, Sinnesorganen, serosen Hauten, Sehnen imd 

 Knochen. 



Thus we find that richness of capillary plexus may occur where 

 the chief factor concerned is the passage of substances through 

 the w^all of the blood-capillary from the lumen outward, as in 

 the kidney ; again where the absorption- of substances is apparently 

 the chief factor, as in the intestine; and again, where removal 



