DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 301 



the large veins, are retained as functional systemic lymphatic 

 trunks in the permanent lymphatic organization. 



Thus Miller (15) finds in embryos of the chick, in the axial 

 preaortal line of the earlier stages, strands and masses of mesen- 

 chymal cells, which differentiate as developing blood cells in situ 

 out of the indifferent mesenchymal syncytium. The anlages of 

 the thoracic duct arise, in close association with these strands 

 of developing blood-islands, as ''isolated lacunae directly from 

 mesenchymal intercellular spaces, are not in any sense derived 

 from the veins, and subsequently coalesce to form the continu- 

 ous channels of the thoracic duct." Miller further states that 

 the blood cells of the earlier axial strands gain access to the 

 mesenchymal spaces constituting the anlages of the thoracic 

 duct, and that finally ''the haemal cellular elements in question 

 reach the blood-stream via the thoracic duct and jugular lymph- 

 sac." 



Miller's observations are of the utmost importance, because 

 they reveal to us, for the first time, the main lymph channel 

 (thoracic duct) of an amniote vertebrate in the haemophoric 

 stage, and establish this stage as a potential type in the lym- 

 phatic ontogeny of any vertebrate embryo. Miller has, in other 

 words, removed the greatest obstacle we have heretofore en- 

 countered in our attempts to establish a correct definition of 

 developing lymphatics in contrast to developing veins. His 

 results prove, I believe, conclusively that in certain vertebrate 

 embryos and in certain stages lymphatic vessels may be actively 

 concerned in transporting blood cells to the general haemal cir- 

 culation, and that 'during this process these haemophoric lym- 

 phatics, still isolated from the general complex of the lymphatic 

 system as a whole, have been mistakenly regarded as venous 

 derivatives and cannot, as a matter of fact, be distinguished, in 

 isolated sections and without reconstruction, from true venous 

 tributaries. I regard this discovery of Miller's and the resultant 

 correction of Sala's earlier work ('00) (24) in the same field, as 

 the most important contribution to the elucidation of the lym- 

 phatic problem which has yet been made. 



