ARE LYMPHATICS TRANSFORMED VEINS 11 



There is no question whatever that venous blood may be 

 present in the lymphatics of the living trout embryo during 

 certain stages of development. The only possible question at 

 issue is the significance which the presence of venous blood in the 

 lymphatics implies. 



III. ARE THE LYMPHATICS OF FISHES TRANSFORMED VEINS? 



The presence of venous blood in the lymphatics has given rise 

 to the view that the lymphatics of fishes are transformed veins 

 and that they are more closely related to the veins than the 

 lymphatics of the higher vertebrates. This view, I think, was 

 originally advanced by Favaro ('06). More recently Mozejko 

 ('13) has advocated the view that a true lymphatic system, simi- 

 lar to that found in mammals, is not even found in fishes but 

 is met with for the first time in the amphibia. 



If the view that the lymphatics of fishes are transformed veins 

 signifies that certain vessels of the embryo which primarily func- 

 tion as veins subsequently assume a lymphatic function, I have 

 positive proof, I believe, in the case of the subocular lymph sacs 

 of the trout, that such is not the case and proof to this effect 

 will, as we shall see, similarly disprove the contention of Mozejko. 

 On the other hand, if the " transformation" theory merely signi- 

 fies that the lymphatics of fishes are derived from the endothe- 

 lium of the veins, that is another question and one which forms the 

 main theme of this paper. As I interpret the "transformation" 

 theory, it signifies that vessels which function primarily as veins 

 may subsequently assume a lymphatic function. 



If the lymphatics of fishes are transformed veins and are 

 more closely related to the veins than the lymphatics of the 

 higher vertebrates, they should invariably function in the ca- 

 pacity of veins, or, at least in the capacity of both veins and 

 lymphatics, at the time of their first appearance in the embryo. 

 This is positively not the case, however, with the subocular lymph 

 sacs of the trout embryo and detailed evidence to this effect will 

 be presented in the following pages. It may be stated here 

 that during the earliest stages of their development the subocular 



