DEVELOPMENT OF THE THORACIC DUCT 61 



of the sac, a short distance cephalad of the jugular approach and 

 of the common jugular venous confluence. 



It extends meso-caudad across the dorso-lateral and dorsal 

 aspects of the internal and common jugular veins between them 

 and the thyro-cervical artery, towards the vagus-sympathetic 

 strand, dipping ventro-mesad between the main vein and the 

 nerves and ending blindly in this situation. 



The process makes its first distinct appea-ance, aft^r the 

 jugular sac has been fully established, in embryos of between 11 

 mm. and 14 mm. crown-rump measure. 



It is noteworthy that individual embryos within these limits 

 show considerable difference in the degree of development of the 

 process. In some it appears earlier, in others a little later, so 

 that in individual instances an embryo of slightly shorter measure 

 may have the thoracic duct approach of the jugular lymph sac 

 further developed than one which exceeds the first in total length 

 by the fraction of a millimeter. 



Thus one of the earliest distinct stages of the process is seen 

 in a 14 mm. embryo (series 210, slide viii, sections 39^42, figs. 



30 to 33 inclusive). 



In this embryo, the thoracic duct approach appears as a short 

 blunt process which projects dorso-mesad from the caudal end 

 of the left jugular lymph sac, a short distance cephalad of the 

 common jugular confluence. Sections 39, 40 and 41 (figs. 30, 



31 and 32, 12} show the earliest stage in the development of this 

 process. In them the area dorsal to the main vein, between the 

 sympathetic nerve and the jugular lymph sac, is occupied, in 

 the direction meso-laterad, by the sympathetic nerve (I}, the 

 termination of the large dorso-medial venous tributary (left 

 sup. intercostal vein) (16), the thyro-cervical artery (24}, and, 

 laterad to this, by the blind terminal portion of the thoracic 

 duct approach (12}, derived from the dorso-medial aspect of the 

 jugular lymph sac (11}. In this embryo the process of the sac 

 ends blindly laterad to the thyro-cervical artery (figs. 33, 34, 24}. 

 It has not yet extended meso-caudad across the dorsal surface of 

 the main vein toward the interval between the thyro-cervical 

 artery and the sympathetic nerve. The thyro-cervical artery 



