THORACIC DUCT DEVELOPMENT IN THE PIG 



443 



slight extravasations. At intervals beyond the farthest extent 

 to which the injection mass has penetrated (X, fig. 30), and 

 located in a line destined to become the pathway of the future 

 thoracic duct, are a number of small blind mesenchymal vacuoles 

 (4s), the largest one of which extends through eight sections at the 

 level of the Ouvierian duct. Being hidden by the veins in a dorsal 

 view of the reconstruction, their positions are indicated on the 

 drawing by dotted circles (4s, fig. 30; see also fig. 13). The con- 

 spicuous size of the lumen of the last space (4s) and the mesen- 

 chymal strand bisecting it, as illustrated in the microphotograph, 

 figure 16, require no further comment. 



1 1 d 





Fig. 17 Transverse section through the thoracic region in a 23 mm. pig embryo 

 (series 23a, J.H.E.C., slide 27, section 18), X 200. 4d, long fusiform lymphatic 

 space in the right thoracic duct line, and tissue bridges traversing its lumen; 

 6b, postcardinal veno-lymphatic surrounded by this space; lid, right postcardinal; 

 13, aorta; 17, oesophagus; 25, venule, branch of the postcardinal. (Reconstruc- 

 tion, fig. 30.) 



