THORACIC DUCT IN THE CHICK 151 



Along the dorso-lateral aspect of the aorta (1) a number of 

 spaces and channels have also developed (fig. 25, 20). In addition, 

 a few smaller lymphatics have appeared along the lateral aspect of 

 the aorta. The dorsal aorta is thus almost completely encircled 

 by a group of spaces and channels comprising the large ventral 

 lymphatic plexus described in the preceding paragraph and the 

 dorsal and lateral sets of lymphatics. 



These encircling lymphatics are in the aggregate homologous 

 with the dorsal set of peri-aortic sinuses in reptiles, as described 

 by Huntington (2). They are also homologous, in all probabil- 

 ity, with the azygos portion of the thoracic duct in the cat, as 

 described by Huntington (3), and in Tragulus, as described by 

 Tilney (20). A comparison with Stromsten's (16) description 

 and figures indicates, too, that the lymphatics associated topo- 

 graphically with the dorsal aorta in the chick are collectively the 

 homologue of the peri-aortic network of h'mph vessels in the 

 loggerhead turtle, or, more specifically, with that portion of 

 the network surrounding the dorsal aorta. In addition, the lym- 

 phatics around the dorsal aorta in the chick, as previously 

 described, may be placed in the same phylogenetic line as the 

 postcardinal and supracardinal divisions of the thoracic duct in 

 the pig, as recently worked out by Kampmeier (15). 



Returning to a further consideration of conditions in the chick 

 at this stage (seven days, fig. 24), it is seen that a few small 

 Ijnmph spaces {17a), also isolated, have appeared along the ventro- 

 lateral aspect of the dorsal aortic roots (2) in the interval between 

 the plexus previously described {17) and the thoracic duct 

 'approach' of the jugular l}Tnph sac {15a). These small spaces 

 represent the beginning of the connection between the thoracic 

 duct 'approach,' on each side, and the unpaired portion of the 

 thoracic duct itself, here composed of the large plexus {17). 



The conditions in the chick thus correspond so closely to those 

 in other forms that it is possible to draw a clear homology between 

 the lymphatics joining the thoracic duct 'approach' to the un- 

 paired portion of the duct itself in the chick and the preazygos 

 segment of the thoracic duct in the cat (Huntington) and Tragu- 

 lus (Tilney), with the cephalic portion of the peri-aortic lymph 



