THORACIC DUCT DEVELOPMENT IN THE PIG 



445 



12d 



4 I = 1 **/ 



6c 



13 



Fig. 18 Transverse section through the region of the mesonephroi in a 21 mm. 

 pig embryo (series 103, slide 32, section 9), X 200. 4, lymphatic spaces or dis- 

 continuous thoracic duct anlagen forming near or against the periaortic veno- 

 lymphatics {6c); 4a, potential lymphatic spaces; 8, dorsal segmental vein; 12d, 12s, 

 right and left supracardinal veins; IS, aorta. 



In series 103 (21 mm.) is foreshadowed the decadence of the 

 periaortic veno-lymphatics and the transference of supremacy to 

 their successors. Frequently throughout their course the veno- 

 lymphatic channels have lost their former fullness and their endo- 

 thelium has been thrown into a slightly wavy and uneven contour, 

 this being especially true of that side of the plexus facing the 

 aorta where these areas of weakness are more abundant and accen- 

 tuated. Coexistent with this condition is an incipient vacuolation 



