22 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



sac of a living trout embryo and the embryo kept alive for some 

 hours, it was found that none of the injecta had passed from the 

 sac into the lateral pharyngeal lymphatic or into the veins. 



The above observations on the living trout embryo are fully 

 borne out by a study of sections and they seem to prove conclu- 

 sively that the subocular lymph sacs of the trout embryo are en- 

 tirely independent of the veins and of the lateral pharyngeal 

 lymphatic during the earliest stages of their development -(figs. 1 

 to 9, inclusive). Injection experiments also prove that the 

 subocular lymph sacs of the trout embryo do not increase in extent 

 in a caudal direction. 



The subocular lymph sacs of the trout therefore furnish us 

 with a striking example of the fact that, not only do independent 

 anlagen of the lymphatic system actually exist, but that they can 

 also be observed and experimented upon in the living embryo. 

 Also, in view of the circumstance that the subocular lymph sacs 

 of the trout do not communicate with the veins during the earliest 

 stages of their development, it is evident that they do not at first 

 function as veins and that they therefore can not be regarded as 

 transformed veins. 



3. The originally independent subocular lymph sacs of the trout 

 embryo establish a temporary communication with the lateral 

 ^pharyngeal lymphatic (fig. 10). 



In steelhead and rainbow trout embryos, developed at a tem- 

 perature of about 10.5C. the subocular sacs establish a direct 

 communication with the lateral pharyngeal lymphatic on ap- 

 proximately the twenty-second day after fertilization. As soon 

 as this communication has been established, the sacs can then be 

 readily injected from the veins by way of the lateral pharyngeal 

 lymphatic, which is the only route by means of which the injecta 

 or blood from the veins can, at any stage of development, reach 

 the subocular lymph sacs. 



If, on account of its independent character, the presence of the 

 subocular lymph sac happened to be unknown, it is evident by 

 injecting into the veins or into the lateral pharyngeal lymphatic, 

 that its presence would still remain undetected until the sac 

 had established its connection with the lateral pharyngeal 



