158 ADAM M. MILLER 



between the lymph sacs and great veins have been formed, the 

 blood cells are admitted to the hemal vascular system. 



In this connection there are two aspects of the question which 

 are especially worthy of consideration. In the first place there 

 are few blood cells within any of the channels forming a part of 

 the thoracic duct system, while in earlier stages, before the tho- 

 racic duct had open communication with the lymph sacs and 

 great veins, many of the lymph channels were filled with the 

 hemal cellular elements. In view of this, and the fact that there 

 are no other means of egress for the blood cells formerly contained 

 in the lymphatics, it must be concluded that the blood cells reach 

 the hemal vascular system via the thoracic duct, the jugular 

 lymph sacs and the recently formed taps between the sacs and 

 the great veins. 



The other feature is the great reduction in the size and number 

 of the extravascular masses of developing blood cells. The dim- 

 inished masses are shown in yellow in figures 27 and 28, the con- 

 ditions in which should be compared with those in figures 22 and 

 24. A few fairly extensive groups are found in the mesenchymal 

 tissue among the channels composing the large ventral plexus 

 [17). These are for the most part quite closely associated with 

 the lymphatics. A few small groups still are found in the region 

 of the thoracic duct 'approach' (fig. 28, 15a). The extensive 

 masses associated in earlier stages with the lymph plexus dorsal 

 to the aortic arches have almost wholly disappeared. This plexus 

 itself is considerably reduced, and it is not unreasonable to assume 

 that the connection between the cephalic end of the plexus and 

 the jugular lymph sac, previously alluded to, is in some way 

 associated with the discharge of the numerous blood cells differen- 

 tiated in this region into the lymph sac. 



In view, therefore, of the intimate relationship between the 

 developing thoracic duct and blood cells in the same general 

 region, and of the sudden and marked reduction in number of 

 these blood cells, both intravascular and extravascular, following 

 the establishment of communication between the duct and the 

 jugular lymph sacs and great veins, the importance of the tho- 

 racic duct as a carrier of hemal cellular elements for a period of 

 embryonic life in the bird can scarcely be doubted. 



