THORACIC DUCT IN THE CHICK 145 



remote from the lymph spaces and channels. In part at least these 

 cells become intravascular when new lymphatics are formed out 

 of the tissue spaces in which they lie and join the general plexus 

 of previously formed vessels. 



Thus far, therefore, the admission of the blood cells to the 

 lymph vessels depends merely upon the topographical relation- 

 ship in the development of the two sets of structures. There are, 

 however, other factors which in all probability enter into this 

 process. Two are of especial interest and importance in the case 

 under consideration. 



It has been pointed out by Dantschakoff that the primitive 

 blood cells in the area vasculosa of the blastoderm and the large 

 mononuclear cells derived from them are capable of ameboid 

 movement. The developing blood cells in the region of the tho- 

 racic duct anlagen are demonstrably of the same type as those in 

 the area vasculosa. Hence it is reasonable to conclude that some 

 of the developing blood cells pass from the tissue spaces into the 

 vessels by virtue of their ameboid character. 



The other factor has hitherto, so far as the writer is aware, 

 been considered only in the study of living tissues. It seems 

 justifiable, however, to extend the conclusions drawn therefrom 

 to fixed tissues. In their study of chick blastoderms in vitro, 

 McWhorter and Whipple (18) have observed the to and fro move- 

 ment, synchronous with the heart-beat, of blood cells not only in 

 the isolated, endothelium-lined spaces which eventually coalesce 

 to form blood vessels but also in the tissue spaces. Further- 

 more, they have observed the entrance of blood cells into the 

 general circulation following then- to and fro movements in the 

 tissue spaces. These phenomena certify the pulsation of the fluid 

 substance in the tissue spaces in response to the heart-beat. It 

 is not uiu-easonable, therefore, to conclude that some of the blood 

 cells in the region of the developing thoracic duct are driven or 

 sucked into the lymph spaces or channels which, as pointed 

 before, open freely into the mesenchymal tissue spaces. 



As a corollary to the phenomena mentioned in the preceding 

 paragraph, an additional factor in the formation of endothelium 

 might be suggested. Granting that the blood cells lying free in 



