THORACIC DUCT DEVELOPMENT IN THE PIG 4C)1 



standing the inability of the eye to discover a connection between 

 these two segments with the aid of high magnifications, it might 

 be urged by those prejudiced that the injection may have been 

 only a partial one. But this objection becomes groundless when 

 the reader recalls that the pressure of the injecting fluid was of 

 sufficient force to produce extravasations, which, as Clark main- 

 tains, signify an excess of pressure in filling the cavity completely ; 

 for he says, "With too great pressure there is produced a mossy 

 appearance around the capillary (lymph) , as has been pointed out 

 by Hover, due evidently to forcing the injection mass through the 

 lymphatic wall." If an opening had been present between these 

 two anlagen the inject ing substance would certainly have obeyed 

 the direction of least resistance and passed into the second one. 

 Nor is the objection valid which would exclude this large blind 

 fusiform space from taking any significant part in the production 

 of the thoracic duct ; for not only is the distinct character and 

 position of this space contrary to such a view but also the fact that 

 the left side discloses similar spaces located in the identical line of 

 the future left duct. Somewhat later embryonic stages make 

 these observations conclusive; for example, in series 192 the post- 

 cardinal segment of the right duct duplicates or agrees in all of its 

 features with that of series 23a, except for its continuity with the 

 anterior or precardinal segment (figs. 30, 31 and 35) and conse- 

 quently with the jugular lymph sac. Moreover, during the prog- 

 ress of his investigation the writer has tentatively assumed the 

 possibility of a centrifugal growing of thoracic duct buds through 

 the large mesenchymal spindle spaces situated only in the thoracic 

 duct pathway, and he has searched for such hypothetical sprouts 

 but has not succeeded in finding a trace of evidence in their favor. 

 Sabin's and Clark's contention that discontinuities in a lym- 

 phatic channel are due to artifacts, resulting during fixation 

 from the unequal shrinkage here and there of its caliber, is easily 

 controverted by the observed facts. In the case of the develop- 

 ing thoracic duct such discontinuities only occur in the stages of 

 the transition phase, in those embryos measuring approximately 

 between 20 and 23 mm. The discontinuous segments or anlagen 

 begin as minute mesenchymal vacuoles which gradually enlarge 



