THORACIC DUCT IN THE CHICK 155 



taps. And inasmuch as the thoracic duct opens into the lymph 

 sac, the latter serves as the portal of entry of the systemic lym- 

 phatics into the venous system, a point upon which emphasis 

 has already been laid by Huntington (2) (3) in his work on rep- 

 tiles and the cat. 



In the embryo of eight daj's the large ventral lymph plexus of 

 the preceding stage (cf. fig. 24, 17) has undergone further coales- 

 cence of its component channels to form a large irregular sinus 

 with a few fenestrae (fig. 26, 17). The sinus lies ventral to the 

 aorta (1) and extends from the junction of the dorsal aortic roots 

 {2) to the level of the exit of the superior mesenteric artery {9). 

 At its cephalic end it branches off into the two slender trunks, 

 one on each side, which extend cephalad and laterad to join the 

 'approaches' {15a) of the lymph sacs {15). These trunks, which 

 are the last components of the Ijrmphatic drainage line to develop, 

 constitute in the bird, according to the anatomical terminology, 

 the right and left thoracic ducts. 



The lymphatics (19) situated between and dorsal to the aortic 

 roots {2) and arches {3, Jf) are here seen to comprise a plexus and 

 a few outlying isolated spaces and channels (cf. fig. 24 and 26, 

 19). This plexus now communicates with the chain of lymphatics 

 {20) lying dorso-lateral to the aorta ( / ) , which in* turn commu- 

 nicate with the large ventral plexus through channels formed by 

 coalescence of spaces lateral to the aorta (cf. fig. 25, 20, 17). 

 The entire group of lymphatics in the region of the dorsal aortic 

 roots and the dorsal aorta as far back as the superior mesenteric 

 artery, with the exception of the mesenteric lymphatics (fig. 26, 

 21), which have not yet joined the thoracic duct, drain into the 

 jugular lymph sacs. 



The masses of developing blood cells were omitted from the 

 reconstruction of the eight day stage (fig. 26). A careful study 

 of the serial sections showed that they were fewer and smaller 

 than in the preceding stage. This may be due to the variability 

 characteristic of the masses, or it may be due to actual reduction 

 of the masses since the blood cells, as stated in in the section on 

 histogenesis, now have access to the jugular lymph sacs through 

 the recent connection established between these structures and 



