THORACIC DUCT DEVELOPMENT IN THE PIG 435 



lymphatic plexus (6a), but has formed a plexus independently 

 of it in a position closely parallel and medial to it. Histogeneti- 

 cally, however, this lymphatic plexus is the same as that of the 

 preceding embryo (series 194, figs. 8, 29), but the redundant 

 vei ous lines of which it seems a shadow picture are still quite 

 regular, although occasional constrictions do suggest their decline 

 and subsequent atrophy. 



The foregoing descriptions of the relation between the develop- 

 ment of lymphatic anlagen and the degeneration of the veno- 

 lymphatics determine clearly that this relationship possesses 

 only secondary significance. These two processes are necessary 

 events in the embryonic history, and if they occur simultaneously 

 and the abandoned venous derivatives occupy a position identical 

 with that of the potential thoracic duct, then the anlagen of the 

 latter will follow the path of least resistance or, better, follow 

 a hydrostatic tendency and collect around their weakened intima 

 and cause its collapse. If, however, these venous lines do not lie 

 in the pathway of the duct, or if their degeneration is slightly 

 retarded so that they are still joined to the systemic blood circula- 

 tion, and are under the influence of its pressure and their intima 

 is still tense, then the lymphatic anlagen will arise independent of 

 any contact with them. 



Fluctuations in the amount of progress attained by the right 

 and left branches of the duct at any given moment during the 

 critical stages of their development are not infrequent; indeed 

 there appears to be a reciprocal action, for when one is large and 

 long, or well represented in the number of its anlagen, the other 

 is only scantily represented. All transition stages show these 

 variations to a greater or lesser degree, but especially favorable 

 examples are series 103 and 191 (21 mm. embryos), which can be 

 regarded as complements of each other, the former being promi- 

 nently dextral, and the latter sinistral in lymphatic growth. In 

 series 103 the right limb of the duct extends as an unbroken chan- 

 nel far back into the postcardinal division of the thoracic region, 

 but the left limb is just visible in its earliest rudiments as a few 

 minute and isolated spaces. In series 191, on the contrary, we 

 meet with a complete reversal of conditions so that the descrip- 



