50 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC 'VESSELS 



In addition, in certain areas, a small group of the earliest lym- 

 phatic anlages appear to develop in the mesenchyme along defi- 

 nite lines, and in distinct patterns, but without any preceding 

 venous reticulum. They impress me, for example, in the area 

 surrounding the omphalomesenteric artery, as systemic lymphatic 

 channels developing ia the placental embryo in regions which are 

 no longer ontogenetically the seat of venous development, al- 

 though occupied by veins in other mammalian types. Thus the 

 cardinal collateral line of the marsupials 3 >J and the correlated 

 venous area of the monotremes 32 no longer develops as a perma- 

 nent veoous plexus in placentalia 34 but only partially appears 

 in certain forms as a temporary and evanescent component of 

 the abdominal venous complex, as described above for certain 

 stages in the development of the cat (pp. 29 to 33 and figs. 20 to 24). 

 Its place, however, is partially occupied by an early lymphatic 

 plexus de\ eloped in the preaortic mesoderm from the omphalo- 

 mesenteric anlages caudad. Here we are apparently dealing with 

 an instance in which general phylogenetic venous lines have been 

 almost or entirely abandoned in favor of other pathways. 

 Such lines appear, however, to be retained under these con- 

 ditions in the lymphatic organization. Thus, the spaces just 

 referred to, as will be shown subsequently, form the first incep- 

 tion of the extensive network of lymphatic vessels which in the 

 adult cat surrounds the aorta and the origin of the superior 

 mesenteric artery, closely interwoven with the semilunar sym- 

 pathetic and the adrenal plexus, and connecting on the one 

 hand with the portal and intestinal lymphatics, and on the other 

 with the beginning of the thoracic duct. This adult mammalian 

 lymphatic plexus forms a perfect lymphatic shadow-picture of 

 the lacertilian ontogenetic peri-omphalomesenteric venous ring. 



In conclusion, I wish to give briefly a summary of my reasons 

 for regarding the structures described in this communication as 

 the anlages of the systemic lymphatic vessels. 



"Except, as recently determined, in Tragulus, in wnich Ungulate the adult 

 postcaval system is of the marsupial type, and in certain embryonic stages of the 

 Cat (15.5-17 mm.) in which the channels appear as evanescent preaortic vessels, 

 subsequently entirely replaced by lymphatics. (Cf. series 258; figs. 20 to 24.) 



