150 ADAM M. MILLER 



most part between a mass of developing blood cells {16) and the 

 vein [10) and ends blindly (see also fig. 19, 16, 15a). A short 

 distance farther caudad are two spaces in the mesenchyme which 

 represent the extreme cephalic end of the rudimentary thoracic 

 duct (fig. 23, 17). These spaces are isolated, for with high magni- 

 fication there cannot be discerned any connection between them 

 and the thoracic duct 'approach' {15a) or the other rudiments of 

 the thoracic duct lying still farther caudad. 



The thoracic duct 'approach' of the jugular lymph sac in the 

 chick is without doubt the homologue of a similar structure" 

 described by Huntington and McClure (9) in the cat. Like the 

 lymph sac itself, it is of venous origin in the chick as in the mam- 

 mal, and forms an integral part of the sac. It arises from some 

 of the more mesially and caudally situated components of the 

 early venous plexus in the region of the jugulo-subclavian angle. 

 When fully formed it extends caudad and mesad for some dis- 

 tance along the mesial aspect of the precaval vein, and terminates 

 blindly (figs. 19 and 23, 15a). Subsequently when it is joined 

 by the thoracic duct, it serves as the portal of entry of this duct 

 into the lymph sac. 



Chick embryo of seven days {Columbia Collection, series no. 512). 

 Reconstruction, ventral view in figure 24-; view from left side, figure 

 25. In this embryo there is considerable advance in the devel- 

 opment of the rudiments of the thoracic duct. The spaces and 

 channels, which in the preceding stage were unconnected, have 

 here coalesced to form an extensive plexus {17) of channels 

 extending from the level of the junction of the dorsal aortic roots 

 {2) nearly to the exit of the superior mesenteric artery. The 

 plexus lies for the most part ventral to the aorta {!), but a few 

 of its components extend around on the lateral aspect of this 

 vessel. Most of the channels in the network are much larger 

 than the original spaces and channels, leaving but small areas of 

 mesenchymal tissue among them; in part they have even fused 

 to form irregular sinuses. The celiac artery (5) in most of its 

 longitudinal course penetrates the plexus {17). The plexus as a 

 whole is still isolated, being connected neither with any part of 

 the hemal vascular sj^stem nor with the thoracic duct 'approach' 

 of the jugular lymph sac {15a). 



