12 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



lymph sacs of the trout embryo function entirely independently 

 of the veins and serve solely as independent reservoirs for the 

 reception of lymph which they receive from the intercellular 

 tissue spaces of the head region. At a later stage of develop- 

 ment, however, these sacs then secondarily establish a communi- 

 cation with the veins through another lymphatic (lateral phar- 

 yngeal lymphatic, 3 in fig. 10) and, at such a time, and then 

 only before lymphatico-venous valves have been formed, can the 

 subocular lymph sacs of the trout receive venous blood from the 

 veins. In view of these observations, those who maintain that 

 the presence of venous blood in the lymphatics of fishes indicates 

 that their lymphatics are transformed veins, or, contend that 

 fishes do not possess a lymphatic system which is comparable 

 to that of other vertebrates, will have to explain why the sub- 

 ocular lymph sacs of the trout embryo first function solely as 

 lymphatics, and may then assume a so-called venous function, 

 only after they have established a secondary and indirect con- 

 nection w r ith the veins. 



As previously stated by Huntington and McClure ('10), I be- 

 lieve that the lymphatics, as distinguished from the veins, are 

 lymphatics from the time of their first appearance in the embryo. 

 My observations on the trout embryo lead me to believe that 

 the passage of venous blood from the veins into the lymphatics 

 is a procedure of no physiological significance whatever and, 

 under normal conditions, is alone made possible by the absence 

 of lymphatico-venous valves. In ordinary circumstances and 

 before lymphatico-venous valves have been formed in the trout 

 embryo, the flow of venous blood into the lymphatics may pos- 

 sibly be related to an intermittent flow of lymph into the veins. 

 In many cases observed in the living trout embryo it appeared 

 to be largely accidental and could be induced by a rough handling 

 of the embryo, by the application of chloretone, or by the shock 

 produced at the time the embryo was placed in the fixing agent. 

 In the absence of lymphatico-venous valves there is no physical 

 impediment to prevent the blood from passing into the lym- 

 phatics from the veins. After these valves have been formed, 

 however, it is extremely difficult to force the injecta into the 



