534 



In order to test the validity of this view the writer has recently 

 undertaken to investigate in detail, by means of wax reconstructions, 

 the development of the main tributaries of the jugular lymph sacs in 

 the cat, which include the thoracic and right lymphatic ducts and the 

 main lymphatic channels which lie along the subclavian, cephalic and 

 external and internal jugular veins. At the present writing I can alone 

 speak definitely concerning the development of the thoracic and right 

 lymphatic ducts since I have been able to study certain embryonic 

 stages, hitherto inaccessible, which, in my estimation, conclusively prove 

 that these two ducts are split ofl" in situ from the veins along their 

 course and that they are not formed independently of the venous 

 system as maintained by Sala and formerly by the writer^) and, 

 furthermore, are not centrifugal outgrowths of the lymph sacs as 

 claimed by Sabin -). 



In order to appreciate fully the conditions in the embryonic stages 

 which just precede and lead up to the definite establishment of the 

 right lymphatic and thoracic ducts, it will be well to describe first the 

 general character and topographical relations of these two ducts in an 

 embryonic stage in which for the first time they have made their ap- 

 pearance as definite, continuous lymph channels which are disconnected 

 from the venous system, except through the mediation of the jugular 

 lymph sacs. Such a stage is met with in an embryo measuring 16 mm 

 in length^). 



The reconstruction of the 16 mm embryo (Fig. 1) clearly shows 

 the relations assumed by the thoracic duct, with respect to the main 

 venous channels which it follows, in the regions which lie in front of 

 and behind the cardinal-Cuvierian junction. It is along the course of 

 these main venous channels, as stated above, that the thoracic and 

 right lymphatic ducts are split off from the veins on the left and right 

 sides of the body, respectively. 



In the 16 mm embryo the thoracic duct opens into the lymph sac 

 slightly in front of the junction between the external and internal 

 jugular veins. As the external jugular vein is situated ventrally it is 



1) Huntington and McCluee, The Development of the Main Lymph 

 Channels of the Cat in their Relations to the Venous System. Anat. 

 Record, No. 3, in American Journal of Anatomy, Vol. 6, 1907. 



2) Sabin, American Journal of Anatomy, Vol. 1, 1902, Vol. 3, 1904 

 and Vol. 4, 1904. 



3) With the possible exception of the 10.7 mm embryo (Harvard 

 Embryological Collection, Series No. 474) the measurements of all em- 

 bryos referred to in this paper were made after fixation. 



