290 GEO. S. HUNTINGTON 



of a centrifugal extension of vascular endothelium, by sprouting 

 or otherwise. Fedorowicz's attempt to argue for a centrifugal 

 extension of his early independent lymphatic anlages has been 

 discussed above. 



3. The resulting lymph hearts establish secondary connections 

 with the adjacent veins. 



4. The independently formed lymphatic mesenchymal spaces 

 and the resulting plexuses contain red blood cells derived from 

 three sources: (a) Some mesenchymal cells contained in the 

 center of a developing lymphatic cavity enlarge, become spheri- 

 cal and are directly transformed into red blood cells, (b) Other 

 red cells develop by the direct transformation of the border endo- 

 thelial cells lining the early lymphatic spaces, (c) The mesen- 

 chyme surrounding the early mesenchymal spaces is actively 

 haemopoetic. These red cells gain access t(3 the lymphatic lumen 

 either by extension of the space formation to include and sur- 

 round the resolving blood-islands (West), or by direct migra- 

 tion of the free red cells through the loose wall of the lymphatic 

 endothelium (Allen, West). 



5. The lymphatic channels receiving free red blood cells from 

 these sources convey them to the venous circulation. They are 

 in this stage haemophoric channels. 



The four papers above quoted agree therefore with the lines 

 laid down by Miller (15, 27) for the development of the avian 

 thoracic duct in the early haemopoetic and haemophoric stages. 

 They all present a striking congruence with the above outlined 

 developmental history of the anterior or jugular lymphsac in 

 mammals, birds and reptiles. 



Returning to the consideration of the conditions obtaining in 

 the mammalian embryo, the outline of the genetic history of the 

 jugular lymphatic sac previously given (pp. 273-280) may be fur- 

 ther supported by the detailed developmental study of one of 

 its main tributary channels, the primitive ulnar lymphatic (figs. 

 12-17). 



With the development of the anterior limb-bud the lateral 

 vein of the body wall receives the blood returning from the 

 extremity through a marginal vessel. The combined trunk then 



