44 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



mesenchymal strands, between which clefts or vesicles — the 

 future lumen of the vessel — ^make their appearance. If one 

 were to follow its development solely by means of the injection 

 method, the superficial facial lymphatic would present the ap- 

 pearance of gradually growing continuously forward from the 

 anterior end of the lateral pharyngeal lymphatic. As a matter 

 of fact, however, such an appearance would only be simulated, 

 since its forward extension, and the extent to which it can be 

 injected, are directly dependent upon the extent to which its 

 independent anlagen have become concrescent with the lateral 

 pharyngeal lymphatic. 



IX. THE DEVELOPMENT OF THE PRECARDINAL (JUGULAR) LYM- 

 PHATICS IN THE TROUT 



In advanced embryonic stages of the trout (fig. 11) and of 

 Amia (fig. 13) the precardinal lymphatics (5) form an extensive 

 system of vessels which communicate indirectly with the veins 

 by means of the lateral and medial pharyngeal lymphatics. 



The development of the precardinal lymphatics parallels that 

 of the lateral pharyngeal and superficial facial lymphatics just 

 described. Discontinuous lymph vesicles make their appearance 

 along the course of the precardinal veins. These discontinuous 

 anlagen subsequently become connected with one another and 

 with the lateral pharyngeal lymphatic, at about the level of the 

 otic lymph sac, to form a system of vessels which can then be 

 injected from the veins. Prior to the connection of these an- 

 lagen with one another and with the lateral pharyngeal lym- 

 phatic, I have been unable to inject them from the veins without 

 producing extravasations. 



The anlagen of the precardinal lymphatics first to make their 

 appearance arise at about the level of the otic lymph sacs. These 

 anlagen (5") are shown in figures 4 to 8, inclusive. Sooner or 

 later (figs. 5, 9 and 10), a communication (7) is invariably estab- 

 lished between them and the lateral pharyngeal lymphatic in the 

 region of the otic communication (13). In a fifteen-day steel- 

 head (fig. 4) a single anlage (5") has made its appearance on the 



