306 Walter A, Baetjer 



of 20 mm., while in the stage between 22 and 23 mm. the uninjected 

 duct occupies the same relative positions. Although this evidence does 

 not seem definite enough to warrant an empirical statement as to the 

 origin, it is certainly suggestive, for such a rapid development could 

 hardly be ascribed to new growth and is therefore most logically ex- 

 plained by assuming the presence of pre-formed channels which then 

 become differentiated, such as will later be shown to be the case in the 

 mesenteric sac. The evidence, then, in connection with the thoracic 

 duct, is that it is preceded by a series of veins from which it is suddenly 

 rather than gradually transformed into lymphatics. The question which 

 seems to me of interest is, Does the receptaculum first form as one of 

 the primitive sacs and the thoracic duct grow from it to meet the lym- 

 phatics growing down from the cervical region, or does the thoracic duct 

 form from a number of segmental anlagen all homologous to the primi- 

 tive cervical or mesenteric ? Dr. Lewis' figures would seem to suggest the 

 second; but, although the primitive duct is not uniform in calibre and 

 contains numerous dilated areas connected by much smaller trunks, I 

 have been unable to find in any series a stage in which these dilated 

 portions exist as independent sacs. However, it should be added that 

 the connecting trunks are, in some cases, so narrow as to suggest this 

 as a possibility. It will thus appear that there is no evidence whatever 

 that the thoracic duct in the pig forms as Sala'^ has described for 

 the chick, 



Sala described the duct in the chick as forming out of solid cords 

 of mesenchyme cells, in which a lumen subsequently developed. Thus, 

 although all the evidence now available seems to indicate the direct 

 venous origin of the thoracic duct, it is not, I think, nearly so conclusive 

 as can be shown in the case of the mesenteric sac. . 



This sac, located in the roof of the mesentery, between the Wolffian 

 bodies, and just ventral to the renal anastomosis of the sub-cardinal 

 veins, was first noticed by Dr. Lewis in following the transformations 

 of the vena cava inferior in rabbits.^ In this paper, in his plate illus- 

 trating the vessels of this region, the lower portions of the sub-cardinal 

 veins are detached from the rest, and "though colored blue, like the 

 veins, they are described as spaces in the mesentery, suggesting the 

 lymph-hearts of the chick." It is also stated that they may be sub- 

 cardinal derivatives. In a later work by the same author on the "De- 



'Sala. Recerche a. lab. di Anat. norm. d. r. Univ. di Roma. A'ol. 7, 1900. 

 'Lewis. Ameb. Jour. Anat., Vol. I. 



