434 ELEANOE LINTON CLARK 



The view presented here that the primitive form of the lym- 

 phatic system is an indifferent capillary plexus and that the 

 main lymph trmiks are formed by the enlargement of certain 

 vessels of this net-work in response to mechanical stimuli of 

 pressure, is of course similar to the results obtained by many 

 investigators with regard to the development of the blood vascu- 

 lar system. Thoma^"* in the chick, found that the formation of 

 the chief arteries and veins was preceded by a stage in which a 

 plexus of simple capillaries was present. He observed that those 

 capillaries more favorably located with regard to the circulatory 

 stream enlarged and became arteries and veins. In discussing 

 this process, Thoma developed a series of 'histomechanical' 

 principles to explain the manner in which this transformation is 

 brought about. That the formation of many of the veins and 

 arteries within the embryo takes place in a similar manner has 

 been shown by Evans^^, and Clark^" has watched the develop- 

 ment of arterioles and venules from a simple capillary plexus in 

 a living tad-pole with a correlated study of the character of the 

 circulation. 



From the present study it seems evident that a similar proc- 

 ess to that described for the blood-vessels takes place in the 

 developing lymphatics. The difference in the character of the 

 flow in the two systems (the definite pulsation in the blood- 

 vascular system and the rapidity of the circulation as com- 

 pared with the flow of lymph, etc.) undoubtedly accounts for 

 many of the final differences in form, for the primitive network 

 of the lymph capillaries closely resembles the non-circulating 

 blood capillary plexus of the yolk sac. 



" R. Thoma, Histogenese und Histomechanik des Gefasssystems. Stuttgart, 

 1893. 



1^ H. M. Evans, On the earliest blood vessels in the anterior limb buds of 

 birds and their relation to the primary subclavian artery. Am. Jour. Anat., 

 vol. 9, no. 2, 1909. On the development of the aortae, cardinal and umbilical 

 veins, and the other blood vessels of vertebrate embryos. Anat. Rec, vol. 3, 

 no. 9, 1909. 



^^ E. R. Clark, Studies of the growth of blood vessels by observations of living 

 tadpoles and by experiments on chick embryos. Proc. Amer. Assoc. Anat., 

 Anat. Rec, vol. 9, no. 1, 1915. 



