THORACIC DUCT DEVELOPMENT IN THE PIG 413 



slightest evidence for 'centrifugal growth' as the fundamental 

 principle in the genesis of the thoracic duct, nor tor its origin 

 from multiple venous anlagen. 



In her most recent paper Sabin 19 takes a position plainly at 

 variance with her earlier view of the development of the thoracic 

 duct, in that she restricts to a considerable degree the importance 

 of centrifugal growth by budding as the active principle or factor 

 in its formation. After briefly mentioning the conditions found 

 by her in two 23 mm. embryos and one measuring 25 mm., she 

 says: 



It is not possible to set limits to the transformation of veins into lym- 

 phatics making the cisterna chyli and thoracic duct, for by comparing 

 the two specimens measuring 23 mm. it can be seen that vessels which are 

 clearly branches of the azygos veins in the one specimen do not seem to 

 connect with the vein in the other. The thoracic duct develops in part 

 as a down growth of the jugular sac and in part, especially its dilated 

 portion or cisterna chyli, as a direct transformation of the branches of 

 the azygos veins. 



This quotation would seem to indicate that she now believes 

 that the longer part of the thoracic duct is produced as a caudal 

 extension of the jugular lymph sac alone, and not, as she formerly 

 held, from two growing sprouts which subsequently meet, one of 

 them derived from the jugular sac, and the other from the cis- 

 terna chyli as an extension cephalad. Her failure to mention 

 either the absence or presence of this last or second sprout, which 

 she claimed to have found in her earlier investigations on the ori- 

 gin of the thoracic duct in human and pig embryos, confirms 

 strongly enough her change of view in this respect. 



IV. OBSERVATIONS AND DISCUSSION 



It is clear now, that there are three distinct views in the field 

 concerning the development of lymphatic vessels: 



1. They spring from the veins at four centers of radiation and 

 by continuous elongation, centrifugal growth and branching 

 invade practically the entire body. 



19 Florence R. Sabin: A critical study of the evidence presented in several 

 recent articles on the development of the lymphatic system. Anat. Rec, vol. 5, 

 no. 9, 1911. 



