THE SUPERFICIAL FACIAL LYMPHATIC 43 



shown in the reconstruction of the 17.5 mm. brook trout (fig. 

 11) and in section in an 18.25 mm. rainbow trout (21, fig. 17). 



It has proved a more difficult matter to follow the development 

 of the superficial facial lymphatic than that of any of the other 

 main lymphatic vessels found in the trout. One reason for this 

 difficulty lies in the circumstance that the superficial facial lym- 

 phatic develops in an extremely loose mesenchyme the cells of 

 which, in the earlier stages of development, cannot be distin- 

 guished from those that line the subocular lymph sac, or, from 

 those that form the walls of the superficial facial lymphatic. 

 Even in the later stages when the superficial facial lymphatic 

 can be easily injected and its presence positively established, it 

 is often quite impossible to distinguish any difference between its 

 endothelial wall and the contiguous mesenchymal cells. It is 

 therefore only with difficulty, and in a most favorable series of 

 sections, that the earliest anlagen of the superficial facial lym- 

 phatic can even be identified at all. 



The earliest anlage of the superficial facial lymphatic that 

 could be identified by the writer may be described as it appeared 

 in a rainbow trout embryo on the twenty-third day after fertili- 

 zation. The anlage could be determined definitely in this par- 

 ticular embryo only upon the left side of the body. On the right 

 side of the embryo, if present, the anlage was indistinguishable 

 in section from a mesenchymal cleft. On the left side of the 

 embryo the anlage of the lateral pharyngeal lymphatic consisted, 

 in part, of a short vessel which communicated with the lateral 

 pharyngeal lymphatic near a point where the latter opened into 

 the subocular lymph sac ; and, in part, of an entirely independent 

 and separate anlage. The former could be followed anteriorly 

 through fourteen sections and is shown in section in figure 37, 

 at the point where it (21) connects with the lateral pharyngeal 

 lymphatic and the subocular lymph sac-(l), and in figure 38 

 eleven sections anterior to the latter. 



In principle, the plan of development of the superficial facial 

 lymphatic is similar to that of other main lymphatic channels. 

 It develops forward into the head region, by the gradual addition 

 to the anterior end of the lateral pharyngeal lymphatic, of 



