THORACIC DUCT IN THE CHICK 157 



In a previous article on the development of the jugular lymph 

 sac in birds (1) the writer stated, in the description of tap forma- 

 tion, that "it is not improbable that a study of later stages will 

 reveal a homologue of the common jugular tap in the mammal" 

 (loc. cit,, p. 486). There is little doubt that the tap on the mesial 

 side of the left jugular vein, referred to in the preceding paragraph, 

 fulfils the requirement. Moreover, the other tap on each side 

 near the jugular-subclavian junction in the chick is in all proba- 

 bility homologous with the jugulo-subclavian tap in the mammal 

 (26), although it is slightly different in position. 



The previously described lymphatics situated dorsal to the 

 aortic roots and arches and dorso-lateral to the aorta here con- 

 stitute a long chain of continuous channels reaching from the 

 level of the jugular lymph sac to the level of the celiac artery 

 (figs. 27 and 28, 19, 20; cf. figs. 24, 25 and 26, 19, 20). At the 

 extreme cephalic end one of the channels of this series opens into 

 the right lymph sac. The probable significance of this opening 

 will be considered in the subsequent discussion of the masses of 

 developing blood cells. The plexus [19) in the region of the aortic 

 arches is considerably reduced as compared with preceding stages. 

 The portion of the dorsal plexus {20) associated with the dorsal 

 aorta communicates with the large ventral plexus {17) through 

 small channels which curve around the lateral aspect of the 

 aorta. It is seen, therefore, that the entire group of lymphatics 

 associated with the aorta and dorsal aortic roots, with the excep- 

 tion of a few still isolated spaces and channels, can now discharge 

 the contents of the channels into the great veins. 



The most interesting and, in fact, from the standpoint of the 

 hemophoric* function of the thoracic duct, the most important 

 feature of this stage is the great reduction in size and number of 

 the masses of developing blood cells. It has been shown in the 

 foregoing pages that the differentiating blood cells associated 

 with the developing lymphatics are admitted or gain access to 

 the lymph spaces and channels and that when the communication 

 is established between the thoracic duct and the jugular lymph 

 sacs the blood cells can thus reach the sacs. Now since the taps 



* Hemophoric — blood bearing or carrying — is a term suggested by Dr. Schulte 

 as a coordinate with hemopoietic — blood producing. 



