34 HENRY MCELDERRY KNOWER 



duct seen under low power appears to lack definite outlines, 

 but is actually a well-defined vessel distinguished from the 

 surrounding tissues of the tail by its filamentous character 

 and ending. The vessel is much the same as that shown in 

 the somewhat more advanced stage of figure 13, and in figure 

 17 (plate 8), under higher power. 



The patterns of the interconnected lymphatic plexuses 

 (anterior, lateral, and dorsal) with the origin of their defini- 

 tive vessels like the similar structures of blood vessels, as 

 explained by Thoma (1893), Mall ('06), Evans ('12), Clark 

 ('18), etc., are determined gradually under the influence of 

 hydrodynamic forces acting between the plexuses and the 

 tissues as development progresses. Hence in the early stages 

 of the dorsal plexus, while communications are relatively free 

 in unrestricted paths, definitive patterns are not to be ex- 

 pected. Variations in the character of early plexuses found 

 in a survey of a large collection of injections are, therefore, 

 to be regarded merely as different expressions of the forces 

 acting on the vessels in establishing a balance. They do, 

 however, furnish instructive examples of the part played by 

 the plexus and the method of formation of larger vessels in 

 interesting agreement with the generally accepted views on 

 these processes in the case of blood vessels well expressed by 

 the following quotations from Evans ('12, p. 586) : 



.... "the tendency to a lingering plexiform type in the main 

 stem and the constant occurrence of the capillary mesh, any 

 part of which may, as it were, be called into service .... 

 gives sufficient choice in the selection of a permanent channel 

 to cause the variations which are so frequent in the adult. ' ' 



and 



.... "a series of elaborate changes must occur before the 

 single vessel is formed. These changes do not involve a fusion 

 process but consist essentially in the selection of one of the 

 possible paths offered by the primitive vessels and the plexus 

 which has sprung up between them. The single definitive 

 vessel may thus be unilateral, median, or even oblique in 

 origin." (Evans, '12, p. 640.) 



