408 RANDOLPH WEST 



jugular lymph sac to designate constituents of the sac which 

 were found at first to contain blood and later to be devoid of 

 blood content. The term veno-lymphatic was simply meant to 

 cover these two conditions of the vessels; for at the time of 

 their studies the criterion of content seemed most available to 

 discriminate between lymphatic and haemal channels. The 

 work of Miller and of Allen demonstrating the i7i situ formation 

 of blood cells and their carriage by lymphatics affords a com- 

 plete and satisfactory explanation of these earlier observations, 

 and Miller's term haemophoric lymphatic satisfactorily de- 

 scribes the actual conditions, and it is to be hoped in interest of 

 clarity will replace veno-lymphatic. This question was fully 

 considered by Huntington (10) at the Thirtieth Session of the 

 American Association of Anatomists. 



The present investigation is concerned with the earliest appear- 

 ance of the posterior lymph hearts in the chick. They are two 

 in number and bilaterally symmetrical. Each one arises in 

 the mesenchyme lateral to the caudal rriuscle plate and posterior 

 to the hind limb bud. Before the lymph heart assumes the form 

 of a single sac-like cavity there exists in this same area a plexus 

 of lymphatic vessels which later coalesce to form the single 

 cavity of the lymph heart. Both the completed lymphatic 

 plexus and later the lymph heart are in connection with several 

 of the most anterior coccygeal veins by means of their lateral 

 branches which pierce the caudal muscle plate, drain the lym- 

 phatics, and then pass outward in the younger embryos to drain 

 a haemal capillary plexus, which bears a superficial relation to 

 the lymphatic plexus. 



It is the purpose of this paper to show that the plexus of 

 lymphatic vessels, which later enters into the formation of the 

 posterior lymph heart arises by the confluence of independent 

 mesenchymal spaces which connect secondarily with the veins; 

 that these spaces are bounded at flrst by mesenchymal cells which 

 become flattened to form an endothelium, and that both in the 

 endothelial lymphatic walls and in the adjacent mesenchyme an 

 active haemopoesis is taking place. 



