THORACIC DUCT DEVELOPMENT IN THE PIG 449 



often become less compact, vesicles and fissures arise in it, and the 

 venous ultima, contiguous to these rarefied tissue-areas, retracts 

 and breaks down ; thus initiating continuity between the originally 

 distinct channels. A section, of such an incipient stage is illus- 

 trated in figure 20 which shows very clearly the vesiculated mes- 

 enchyme (Jf) in and amongst the veno-lymphatic plexus (6c). 

 Typical examples of extra-intimal replacement also become very 

 abundant and are significant as confirming the evidence already 

 given for the reorganization of the intima whenever a lymphatic 

 channel appropriates the pathways of redundant veins or venules. 

 A photograph (fig. 21) of such a condition in a 22 mm. embryo 

 describes more plainly than an extended narrative the impoitant 

 features in the transition period of the ciserna chyli. The com- 

 partments or loculi of the potential cisterna are traversed by dis- 

 tinct, delicate and devious lines which upon closer examination 

 are found to be composed of compressed or scale-like cells, placed 

 end to end, and to represent the discarded endothelium of the 

 former venous derivatives (6c). This is shown when the endothe- 

 lium is followed either forward or backward and it can. be found 

 occasionally lining a sharply defined cavity containing blood and 

 then again to be pushed far into the lumen apparently by the 

 pressure of the fluid within, large mesenchymal spaces (4-) on its 

 external surface. At still other places where fusion of several 

 parallel channels has occurred simultaneously, this evanescent 

 venous intima is visible in cross-section as torn fibrils pendant 

 from the irregular and frayed walls, or lying isolated in the lumen 

 of the new or compound cavity. Besides these vestiges of the 

 venous intima there are broader and thicker shreds of tissue, 

 which are composed of a mass of ordinary mesenchymal cells 

 jutting into the cavity and which indicate therefore the position 

 of former boundaries between separate channels. Examination 

 of the sections will also show distinctly that the outlines of the 

 perivenous spaces are ill-defined and radically unlike those of the 

 venous channels which they surround. In the confines of the 

 transitional cistern a-anlage the irregular elliptical or cuboidal 

 mesenchymal cell is the prevailing type and exists in strong con- 

 trast with the flattened and dense endothelial cell of a normal 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13, NO. 4 



