Mesenteric Sac and Thoracic Duct in Embryo Pig 305 



in the three areas mentioned above — an embryo can be found (between 

 21 and 23 mm. in length) with lymphatics in the cervical region and 

 in the mesentery before there is any evidence of similar vessels in the 

 location of the future thoracic duct. At this stage and in all the suc- 

 ceeding ones up to 23 mm., all of the vessels dorsal to the aorta can be 

 completely injected through the umbilical artery or vein, though the 

 branches of the azygos veins are in the exact location of the thoracic 

 duct which replaces them in the later stages from 23 mm. upwards. 

 Below 21 mm., true lymphatics exist as such only in the cervical region, 

 the mesenteric sac being represented by a plexus of capillaries in free 

 communication with the veins. This is well shown in Figs. 1 to 4, 

 which represent sections through the mesenteric sac. The capillaries at 

 these stages are all engorged with blood and injection mass, and in all 

 the sections large openings into the veins can be easily made out. 



Before proceeding, however, to the consideration of the origin of this 

 sac, which is indeed the principal object of this paper, several points 

 may be set forth concerning the origin of the thoracic duct proper. 

 This work, however, which is still incomplete, is to be the subject of a 

 future article; therefore, only those points will be stated here which 

 have a direct bearing on the main subject of this paper. 



It has been shown that the lymphatics in the cervical region are 

 derived directly from the veins, and this conclusion leads us naturally 

 to expect that the primitive thoracic duct and mesenteric sac would in 

 all probability be found to have a similar origin. This seems to be 

 definitely established for the mesenteric sac; but for the thoracic duct, 

 though all the evidence now at hand seems to favor a like origin for this 

 branch of the lymphatic system, still it does not as yet seem as conclu- 

 sive as could be desired. 



The thoracic duct proper, or the part of the lymphatic system dorsal 

 to the aorta, is not seen until the mesenteric sac is almost completely 

 differentiated from the veins, this area being occupied by numerous 

 branches of the azygos veins, all of which can be completely injected. 

 Lewis, in his work on the development of the lymphatics, concludes that 

 "lymphatic vessels develop along the course of the azygos veins appar- 

 ently from independent venous outgrowths, all of which unite to form a 

 continuous system, later acquiring new and permanent openings into 

 the veins." The evidence which could be gathered from my own series, 

 I think, in general, supports this conclusion, for sections taken from the 

 same levels show all the vessels dorsal to the aorta injected in an embryo 



