GROWTH OF BLOOD-VESSELS IN FROG LARVAE 43 



a thin layer of fluid next the wall which is immovable. His 

 explanation for growth in size of vessels is that it is brought 

 about through the agency of the vasomotor nervous mechanism; 

 that, following increased metabolism and formation of new 

 capillaries, there is a reflex widening of the arteries and possibly 

 also of the veins of the affected region. This widening, if con- 

 tinued long enough, results in a permanent adaptation of the 

 vessel wall to the increased volume of blood by growth processes. 



Roux apparently agrees with Thoma's law as to the increase 

 in thickness of the vessel wall. 



Mall ('00), in an extensive review and discussion of Thoma's 

 histomechanical laws, finds support for Thoma's first law, in 

 his studies on the growth of glands. Like Roux, however, he 

 disagrees with Thoma in his hypothesis that the formation of 

 capillaries is dependent on increase in blood-pressure in the capil- 

 lary area. ''In reality," he says (p. 250), ''we can only state 

 definitely that with the new formation of tissue new blood-ves- 

 sels may grow into it, for all new tissue does not have blood- 

 vessels." The precise stimulus for the formation of capillaries 

 is unknown. Again (p. 251), he says, "The first and guiding 

 blood-vessel is the capillary, which grows in all directions, form- 

 ing a plexus. Secondary changes made arteries and veins of 

 them and their laws of growth have been discovered and clearly 

 stated by Thoma." 



It has been shown by a series of investigators — among them — 

 Erick Miiller ('03, '04), Rabl ('07), Bremer ('12) and H. Smith 

 ('09), and particularly Evans ('09, A and B) that many of the 

 larger arteries and veins in the body of the developing embryo 

 are first formed as capillaries, which grow as irregular plexuses, 

 and out of which certain ones are differentiated to form arteries 

 and veins. Evans, who has made the most extensive studies in 

 this field, has described the caudal portion of the aorta, the 

 chief veins, the pulmonary, subclavian and sciatic arteries as 

 developing in this manner. He concludes that the histomechan- 

 cal laws of Thoma are the factors which govern the process. 



A number of investigators have suggested that new capillaries 

 are formed as the result of the action of specific 'chemiotactic' 



