MODIFICATIONS WHICH GROUND PLAN UNDERGOES 15 



detachment and separation of the subocular lymph sacs from the 

 rest of the lymphatic system, and (3) to the forward extension 

 of the lateral pharyngeal lymphatic into the head region as the 

 superficial facial lymphatic, which aids in draining this region in 

 place of the subocular lymph sac. These modifications are shown 

 in figure 11 which represents a reconstruction of the principal 

 lymphatics, veins and arteries found on the left side of the head 

 and pharynx in a 17.5 mm. brook trout embryo. 



In the 17.5 mm. brook trout embryo (fig. 11) the precardinal 

 lymphatics (5) form a much more extensive system of vessels 

 than in the rainbow trout on the twenty-second day (fig. 10). 

 They drain the gill arches, the walls of the pericardial cavity 

 and the dorsal mesentery, and communicate with venous circu- 

 lation through the medial and lateral pharyngeal lymphatics. 

 As shown by the reconstruction, the lateral pharyngeal lymphatic 

 (3) of the left side has lost its original connection with the lym- 

 phatic of the lateral line of the trunk (10) in the 17.5 mm. brook 

 trout embryo. The result is that the lymphatic drainage of the 

 head and pharynx is now independent of that of the lymphatic 

 of the lateral line of the trunk and enters the venous circulation 

 only by way of the medial pharyngeal communication (2) and, 

 to a lesser extent, at a point near the caudal end of the otocyst, 

 where the lymphatics also communicate with the veins (13, 

 otic communication). The medial pharyngeal communication 

 appears to be retained in still later stages and in the adult 

 trout, as the principal point at which the lymphatic drainage 

 of the head and pharynx enters the venous circulation. 



It has been stated above that the left lateral pharyngeal 

 lymphatic has lost its connection with the lymphatic of the lateral 

 line of the trunk and, in this 17.5 mm. brook trout embryo, there 

 seems to be no doubt that such is the case. I have evidence, 

 however, which seems to show that the lateral pharyngeal 

 lymphatic may not always establish a connection with the veins 

 on both sides of the embryo, either at the cardino-Cuvierian 

 junction, or near the caudal end of the otocyst (otic communica- 

 tion). It is evident, therefore, that variations found in ad- 

 vanced stages cannot always be attributed to the loss of com- 



