THE MEDIAL PHARYNGEAL LYMPHATIC 47 



maintained and the theory that lymphatic endothelium, in gen- 

 eral, is of venous origin would then necessarily fall completely 

 to the ground. 



X. THE DEVELOPMENT OF THE MEDIAL PHARYNGEAL 



LYMPHATIC 



The earliest continuous vessel in the trout embryo which con- 

 veys lymph from the head and pharynx to the venous circulation 

 is the lateral pharyngeal lymphatic, which may communicate 

 with the veins in the cardino-Cuvierian and otic districts of 

 communication. Although the otic communication is wanting, 

 this stage is represented on the left side of the twenty-two-day 

 rainbow trout embryo (right side of fig. 10). This early plan of 

 drainage soon becomes modified in the trout embryo, however, 

 by the formation of a new lymphatic chaiinel, the medial pharyn- 

 geal lymphatic (4, fig. 10). This vessel taps the lateral pharyn- 

 geal lymphatic secondarily and then directs the lymph from the 

 latter to the precardinal (jugular) vein (6),' with which it com- 

 municates at a point near the exit of this vein from the cranial 

 cavity (2, medial pharyngeal communication, left side of fig 10). 

 This modified plan of drainage has been established on the 

 right side of the twenty-two-day rainbow trout embryo (left side 

 of fig. 10; see also 4 in fig. 11). 



From what has been stated above, it is evident that the medial 

 pharyngeal lymphatic makes its appearance in the trout embryo 

 at a relatively late stage of development, and that it then serves 

 only in the capacity of conveying to the venous circulation by an 

 additional route, lymph originally collected by an independently 

 formed system of lymphatics (subocular lymph sac and lateral 

 pharyngeal lymphatic) . 



In a series of rainbow trout embryos which were developed at a 

 temperature of about 10.5°C, the medial pharyngeal lymphatic 

 made its first appearance between the twentieth and twenty-first 

 days. On the twenty-first day (4, fig. 9) it was found to be pres- 

 ent on both sides of the embryo and to be connected with the 

 veins at the medial pharyngeal communication (2, fig. 9). 



