THORACIC DUCT IN THE CHICK 149 



The masses of developing blood cells in this embryo are un- 

 usually scarce. A few small groups (16) are situated in the mesen- 

 chymal tissue ventral to the aorta (1), and three larger groups 

 (also marked 16) are seen between the dorsal aortic root (2) and 

 the jugular vein (10). 



Chick embryo of six days and twenty-two hours, 16 mm. (Columbia 

 Collection, series no. 463). Reconstruction, ventral view. Figure 

 22. In this embryo the lymphatics (17) are in approximately 

 the same stage of development as in the preceding embryo. They 

 are situated for the most part along the ventral aspect of the 

 aorta (1). A few spaces are situated ventro-lateral to the dorsal 

 aortic roots (2). 



The masses of developing blood cells (16) are much more ex- 

 tensive than in the preceding embryos of about the same stage, 

 thus exhibiting the variability of the structures. The groups 

 associated with the rudimentary thoracic duct (17) lie ventro- 

 lateral to the aorta (1), with some tendency to cluster around 

 the celiac artery (5) . The rest of the groups extend in a continu- 

 ous mass on each side along the ventro-lateral aspect of the dorsal 

 aortic roots (2) and along the mesial aspect of the jugular veins 

 (10) (cf. fig. 6, 16) as far forward as the fourth aortic arch (4)- 

 A large aggregation of developing blood cells (16a) also lies 

 between and somewhat dorsal to the aortic roots (2) and the arches 

 (3, 4) (cf. fig. 6, 16a), being connected with the left lateral groups 

 by cords extending ventral and dorsal to the roots and arches. 

 Associated with this large mass are a few lymphatics (19) (cf. 

 fig. 6, 19) which subsequently join the thoracic duct and may also 

 communicate with the cephalic end of the jugular lymph sac. 



One of the most interesting features of this particular stage is 

 the well developed thoracic duct ^approach' of the jugular lymph 

 sac (fig. 22, 15a). A reconstruction to show this structure was 

 made on a larger scale and is illustrated in figure 23. The lymph 

 sac iself (13) lies dorsal to the jugular vein (10) and fits into the 

 angle between the latter and the subclavian vein (11). The tho- 

 racic duct 'approach' (15a) is situated on the mesial aspect of the 

 j ugular-subclavian angle and thence extends a short distance 

 caudad along the mesial side of the precaval vein. It lies for the 



