ARE LYMPHATICS TRANSFORMED VEINS 13 



lymphatics from the veins without causing extravasations, and 

 I therefore doubt if the flow of blood through valves which are 

 especially constructed to prevent its passage can be regarded at any 

 time, either in fishes or in any other vertebrate, as a normal pro- 

 cedure. It is true that blood corpuscles are often found in the 

 lymphatics of the trout after the lymphatic o- venous valves have 

 been established. In such cases, however, if these corpuscles 

 have not been forced through the valves under unusual condi- 

 tions, it is likely that they are either the remains of those which 

 passed into the lymphatics before valves were formed, or cor- 

 puscles which the lymphatics have taken up from the tissue 

 spaces. 



Although I have not had the opportunity of studying the living 

 embryos of Amia and Lepidosteus, I can state from a study of 

 sections that, before lymphatico-venous valves have been formed, 

 venous blood flows into the lymphatics from the veins. After 

 the formation of valves, however, the blood no longer has free 

 access to the lymphatics and, in this respect, the conditions found 

 in the embryos of ganoids are the same as those in the trout. 



It must be borne in mind that the passage of blood from the 

 veins into the lymphatics is a phenomenon not exclusively confined 

 to the embryos of fishes. In the embryos of all other vertebrates, 

 including mammals, blood must necessarily have free access to 

 the lyrnphatics during those early stages of development in which 

 lymphatico-venous valves are wanting and in which the lym- 

 phatics are capable of being freely injected from the veins. 

 Since the passage of venous blood into the lymphatics normally 

 ceases with the formation of lymphatico-venous valves, it is 

 hardly probable that its presence in the lymphatics of the em- 

 bryo, in either fish or mammal, is indicative of any physiological 

 condition which is essential to the economy of the vascular sys- 

 tem at a time before, but not after lymphatico-venous valves 

 have been formed. It is quite possible in the case of some fishes 

 that lymphatico-venous valves may never be formed. Even in 

 such cases, I believe that the influx of venous blood into the 

 lymphatics would possess no physiological significance beyond 

 that temporarily displayed in the embryos of vertebrates in gen- 



