THORACIC DUCT IN THE CHICK 161 



disappearance of the nucleoli and the rearrangement of the 

 chromatin into a heavy reticulum. 



These developing blood cells, at first scattered in the mesen-- 

 chymal intercellular spaces, become aggregated, following the 

 increase in their number, into extensive masses which lie along 

 the line of the thoracic duct and also in the region dorsal to the 

 aortic arches. 



When the lymphatics comprising the rudiments of the thoracic 

 duct develop, some of the developing blood cells, even some of 

 the smaller groups, are seen to be contained within them, while 

 others are still free in the mesenchymal intercellular spaces, that 

 is, extravascular. Subsequently more and more of the cells are 

 observed to be intravascular. It may be concluded that the 

 developing blood cells become intravascular by simple inclusion 

 as the lymph channels develop, or by virtue of their ameboid 

 character, or as a result of their motion to and fro in the tissue 

 fluid in response to the heart-beat. 



The blood cells that are admitted to the lymph channels con- 

 stituting the thoracic duct system during its development rapidly 

 diminish in number after communication is established between 

 the thoracic duct and the jugular lymph sacs and between the latter 

 and the great veins. It can be inferred then that the blood cells 

 which develop along the line of the thoracic duct reach the blood 

 stream via this duct and the lymph sacs. Considering the vast 

 number of hemal cellular elements, especially erythrocytes, aris- 

 ing in this region and the probability that they reach the general 

 circulation via the thoracic duct, this duct assumes an additional 

 phase of importance in the chick in that it performs a hemophoric, 

 or blood carrying, function. 



In conclusion, I wish to thank Dr. Huntington and Dr. Schulte 

 for their valuable criticism and suggestions. Dr. McWhorter for 

 his painstaking work in making the photomicrographs, and Mr. 

 Petersen for his careful execution of the color plates. 



