262 GEO. S. HUNTINGTON 



spaces situated beyond the furthest point reached by the success- 

 ful complete injection. Such spaces, if they exist and if they 

 are added progressively by accretion to the already connected 

 segment of the developing lymphatic channels, will in the suc- 

 ceeding stages give to the injected area the appear ance of a 

 continuous centrifugal extension toward the periphery. This 

 appearance is, however, absolutely deceptive and the opposite 

 of the actual genetic process involved in the further growth of 

 the systemic lymphatic channels. McClure's (4) and Kamp- 

 meier's (3) reexamination of one of the 'completely' injected 

 pig-embryos of the Johns Hopkins University collection (Series 

 23A) in serial sections demonstrates unequivocally that beyond 

 the utmost limit of a 'complete' injection of the patent proximal 

 portion of the thoracic duct, independent endothelial-lined spaces 

 exist in the path of the advancing duct, which in the course of 

 further development would have formed increments to its exten- 

 sion, and would, in a slightly more advanced stage, have been 

 included in the injectible area. 



While these self-evident facts hardly seem to demand further 

 explanation or illustration, yet my own personal experience has 

 convinced me of the difficulty of conveying the details of lym- 

 phatic development to readers only moderately or insufficiently 

 familiar with the appearance of developing lymphatic areas. 

 The important point is to narrow the inquiry to a small number 

 of ascertainable facts. If these facts can be definitely estab- 

 lished then correct logical deductions can be based on them. 

 Figure 1 illustrates diagraimnatically a stage (14 mm.) in em- 

 bryos of the cat in which the jugular lymphsac {6) has been 

 formed with its double lymphatico-venous connection, at the 

 common jugular {8) and jugulo-subclavian venous angles {9), 

 and its thoracic duct approach (7). An injection of this portion 

 of the future lymphatic apparatus would correspond to the area 

 shown in stipple in figure 1. The independent lymphatic an- 

 lages (cross-shaded in fig. 1) lying caudal to the thoracic duct 

 approach, in the line to be occupied by the future duct {10), 

 would not be revealed by the injection, although their pres'ence 

 can be determined in serial sections of the injected embryo. 



