THORACIC DUCT DEVELOPMENT IN THE PIG 



425 



Fig. 5 Transverse section through the left lower cervical region in a 19 mm. 

 pig embryo (series 168, slide 17, section 9), X 150. 1, posterior tip of the jugular 

 lymph sac; 2, thoracic duct approach; 4. lymphatic space formed against the intima 

 of a precardinal veno-lymphatic, (6a); 9, internal jugular; 13, wall of the aorta; 

 14, sympathetic trunk; 15, vagus; 16, recurrent laryngeal nerve; 17, oesophagus. 

 (Reconstruction, fig. 28.) 



tial thoracic duct there is a distinct vacuolation of the tissue as 

 shown in figure 6 (4) , medial to a veno-lymphatic tributary (6 a) . 

 Although exceedingly difficult to describe, these vacuoles or 

 spaces are seen to stand out conspicuously, perhaps by the greater 

 clearness of their cavities, even if their boundaries are ill-defined, 

 and by their preponderance in size over the more regularly dis - 

 posed openings of the ground substance. In a section taken fur- 

 ther forward another lymphatic anlage occurs as a large lenticular 

 space (4, hg- 5) against the intima of a retrogressive veno-lymphatic 

 (6a), but it can be distinguished definitely only in two sections. 

 In the same figure the thoracic duct approach (2) is indicated. 

 This ends blindly but is followed shortly by large clear-cut spaces 

 (4) which are in no way continuous with it (fig. 28). Similar 

 and widely separated anlagen occur along the anterior or upper 

 half of the oblique vessel (7) and immediately ventral to it. 



