460 OTTO F. KAMPMEIER 



Fourthly, these mesenchymal perivascular spaces may have fused 

 into a profuse plexus and become widely open to the jugular 

 lymph sac as in the case of series 194 (fig. 29), but they, as yet, do 

 not show a well-defined or specialized wall (figs. 8 and 9). In the 

 fifth place, all of the discontinuous mesenchymal spaces follow 

 one another in a succession practically undeviating which repre- 

 sents an outline or fragmentary picture of the future duct. Out- 

 side of this line there are no lymphatic spaces. Sixthly, in the 

 third or lymphatic phase, when continuity of the duct and its 

 branches has been established, no perivenous or other isolated 

 vacuities can be discovered. If the discontinuous lymphatic 

 anlagen were artifacts we should expect to find the largest num- 

 ber of them in this last phase because the diameter and the bulk 

 of the embryos are greater, and therefore the longer time required 

 for the fixing fluid to penetrate to their centers would make 

 possible greater uneveness of fixation and consequently greater 

 shrinkage. 



Because the elongation of the thoracic duct is effected by a pro- 

 gressive summation or centripetal addition of large mesenchymal 

 spaces to that part of the anlage already confluent with the lymph 

 sacs, the injection of successive transition stages up to the time 

 when continuity has been acquired throughout its entire course 

 will show a gradual increase in the length to which the injection 

 mass has penetrated; but the study of serial sections will also 

 reveal anlagen which lie beyond the farthest point of the injec- 

 tion and are inaccessible to it on account of their discontinuity, 

 or because they have not as yet become confluent with the anlage 

 into which the injecta were introduced. 



This leads to a second contention of Sabin, namely, that the 

 study of serial sections alone is inadequate, and that continuity 

 of the apparently discontinuous lymphatic anlagen can be demon- 

 strated by complete injection. A more radical refutation of this 

 argument than that furnished by her own series 23a is scarcely 

 possible. The abrupt break between the precardinal injected 

 segment of the right thoracic duct anlage in this embryo and the 

 postcardinal uninjected segment (fig. 31) bears out in a striking 

 manner the evidence derived from the writer's series. Notwith- 



