38 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



Fig. 25 (series 241, slide xxx, section 4) gives, in a magnifi- 

 cation of 75 diameters, a topographical view of the entire field. 

 This stage, compared with the preceding 17mm. embryo, is marked 

 by the full development of the supracardinal venous line, respon- 

 sible for the production of the greater portion of the typical 

 adult placental postcava below the renal level, and by the cor- 

 related development of the supra- or retro-aortic lymphatic 

 sinuses associated with the same. 



The periaortic area in fig. 25 gives a clear view of the vascular 

 relations and of the postcardinal and supracardinal axial venous 

 trunks. 



The former (67, 68,} are seen on each side, between aorta (7) 

 and metanephros (65), receiving the veins from the mesonephroi 

 in whose dorso-medial border they are lodged. 



The latter (59, 60) lie dorsal to the aorta (7) between this ves- 

 sel and the sympathetic strands (1). 



The right supracardinal (60) has already gained the ascendency 

 and is in process of establishing the channel of a normal right 

 retro-aortic postcava, which is the typical vein for the cat. The 

 correspondingly reduced left supracardinal (59) occupies the same 

 situation on the left side. Associated with the supracardinal 

 venous channels are the supracardinal lymphatic trunks (76), 

 which form the anlages of the main adult retro-aortic lymphatic 

 plexus. These develop as extraintimal spaces replacing portions 

 of the earlier supracardinal venous reticulum. In accordance 

 with the normal type of development observed in this individual 

 embryo, the large permanent supracardinal (postcaval) vein of 

 the right side is accompanied by a relatively small lymphatic 

 channel (76 right), while on the left side the much reduced left 

 supracardinal (59) is already nearly replaced by the correspond- 

 ing lymphatic vessel (76 left). In course of further normal devel- 

 opment this replacement will become complete and then the 

 area formerly occupied by the left supracardinal vein will be 

 entirely filled by the substituted large left retro-aortic lymphatic. 

 The permanent functional venous channel of the right side (60) 

 on the other hand, developing into the typical placental post- 

 cava, will be accompanied by a relatively small right lymphatic 



