DEVELOPMENT OF THE THORACIC DUCT 63 



across the main vein. In fig. 36 (section 16) a blind terminal 

 protrusion (12) separates from the main lumen of the sac (11), 

 indicating the frequent bifid character of the thoracic duct ap- 

 proach. The succeeding sections (18 and 20, figs. 37 and 38) 

 hence show an apparent recession of the entire process. In sec- 

 tion 22 (fig. 39) the approach has again extended mesad toward 

 the thyro-cervical artery and the superior intercostal vein. In 

 the succeeding section (fig. 40) the fundus of the process terminates 

 for the second time in a blind pouch-like protrusion (12). In the 

 following section (24, fig. 41) the approach (12) again appears 

 fully developed and in open connection with the main jugular 

 lymph sac (11). It maintains the character in section 25, and 

 in section 26 (fig. 42) the terminal of this second protrusion (12) 

 ends blindh^ It does not reappear in the succeeding sections. 



We are dealing here with an instance of very common occur- 

 rence, in which the thoracic duct approach of these earlier stages 

 not only gives off a number of blind terminal diverticula, but in 

 which the entire approach arises by two or three distinct and 

 separate origins from the main jugular lymph sac. This character 

 will again be discussed below (cf. p. 66). in connection with the 

 interpretation of certain adult conditions, but the instance now 

 under discussion shows the early type of this variation very clearly, 

 so that in a graphic reconstruction the thoracic duct approach 

 of this embryo would appear as in figure on page 64. 



This embryo therefore presents for its age an unusually long 

 and well developed thoracic duct approach, with triple terminal 

 pouches, extending through sixteen sections, while at the same 

 time it remains throughout this extensive course in the early 

 position, viz.: lateral to the thyro-cervical artery (24) and the 

 common trunk of the dorsal and dorso-medial tributaries of the 

 internal jugular vein (16, 17, 46). The approach has not as yet 

 arched mesad into the neuro-venous recess, and is hence in 

 marked contrast to the succeeding stages presently to be described. 



In a 13.5 mm. embryo, series 189, the development of the tho- 

 racic duct approach has gone much further, although the embryo, 

 a litter mate of series 210, measures half a millimeter less. 



The thoracic duct approach of the left jugular lymph sac 



