THE EMBRYONIC GROUND PLAN 9 



mentioned above (9 and 13), constitute all of the typical points 

 of communication, as far as I can determine, which are estab- 

 lished in the trout embryo, in the regions of the head and pharynx, 

 between the lymphatics and the veins. 



4. The precardinal or jugular lymphatics (5 and 5' in figure 10, 

 truncus lymphaticus precardinalis vel jugularis) 



The precardinal lymphatics are not completely established in 

 the form of a continuous system of channels on the twenty-second 

 day. They follow the course of the precardinal veins and, in the 

 twenty-two-day rainbow trout embryo, are represented by sev- 

 eral independent and discontinuous lymph vesicles or sacs (5' in 

 fig. 10) and by two irregularly-shaped sacs (5 in fig. 10). The 

 latter lie in the region of the three posterior efferent aortic arches 

 (17, 18 and 19, fig. 10), and communicate with the lateral pharyn-^ 

 geal lymphatic about opposite the caudal end of the otocyst (7 

 in fig. 10). At a slightly later stage of development the pre- 

 cardinal lymphatics form a continuous system of vessels which 

 drain into the veins through the lateral and medial pharyngeal 

 lymphatics. 



The above constitute the main or principal continuous lym- 

 phatic channels which are found in the steelhead and rainbow 

 trout embryo on the twenty-second day after fertilization, and 

 also in the brook trout at a corresponding stage. 



If one injects into the caudal vein at this stage of develop- 

 ment, on account of the absence of lymphatico-venous valves, 

 the injecta will pass freely into the lymphatics from the veins 

 through any one or all of the typical points of communication 

 which exist between the two. It will therefore fill up com- 

 pletely, on each side of the body, the lateral pharyngeal lym- 

 phatic, the subocular lymph sac, the lymphatic of the lateral line 

 of the trunk, the medial pharyngeal lymphatic and that portion 

 of the precardinal lymphatic (5) which has established a communi- 

 cation with the lateral pharyngeal lymphatic near the caudal end 

 of the otocyst. The injecta will not pass into the independent 



