LYMPH-FLOW AND LYMPHATICS, CHICK EMBRYOS 439 



conditions in the lymphatics and in the veins with which they 

 connect. 



It seems reasonable to assume that somewhat similar conditions 

 are present during the early development of the Ijrmphatic 

 system in other vertebrates and that the variations in the num- 

 ber and position of the lymph sacs as well as the difference in 

 pattern of the lymph vessels in various forms are associated with 

 differences in the condition and direction of the lymph flow. 



It appears probable, from the descriptions of many investi- 

 gators, that the jugular lymph sac of mammals develops and 

 attains a large size at an earlier period than the lymph sacs of 

 the chick. In this connection I should like to mention again the 

 enlargements of the superficial lymphatics just anterior to the 

 leg, which occur normally in embryos of 6 days. These are 

 sac-like in form and compared with the other lymphatics of that 

 stage, they are very large. These 'sacs' occur in a loose tissue 

 at a stage of hesitation when the lymph-flow, whose direction 

 in this region had been anterior for several hours, is about to shift 

 and move posteriorly. Moreover, in the case of the chick with 

 the stunted tail, described above, whose lymph heart was small 

 and inefficient, large sacs were formed in the posterior pelvic 

 region fully twenty-four hours earlier than the time at which 

 they normally appear. These two examples show that if the 

 right mechanical conditions are present a sac may form at an 

 early stage in the chick. In the case of the jugular lymph sac of 

 mammals such conditions probably make their appearance 

 earlier than in the chick. It should also be remembered that 

 before it was known that the primary lymphatic system con- 

 tained blood and could be studied in the living and injected 

 directly, the early lymphatics were constantly mistaken for 

 blood-vessels or overlooked altogether. It is therefore possible 

 that more extensive lymphatic plexuses than have hitherto been 

 described may be present in mammalian embryos before the 

 jugular plexus has become transformed into a sac. 



The view that the lymph sacs of embryos represent temporary 

 reservoirs of lymph and that they are isolated portions of a develop- 

 ing lymphatic system which has not yet become continuous and is 



