THORACIC DUCT DEVELOPMENT IN THE PIG 431 



example, the endothelial lining of the jugular vein (9, fig. 9) 

 stains clearly and is sharply defined, and there are no spaces 

 external to it. 



Extra-intimal replacement occurs only among those venous 

 channels which are immediate antecedents of lymphatics in time 

 and place. This is attested by the fact that other veins in their 

 atrophy are not surrounded by spaces but disappear by the grad- 

 ual reduction of their caliber, or by a process of constriction 

 cutting the channel into segments which become smaller and 

 smaller to form dense masses or islands of cells ultimately to be 

 lost in the mesenchyme. As an instance of such a process may be 

 described the reduction and dismemberment of a large portion of 

 the plexuses uniting the original supracardinal lines {25, figs. 28, 

 29 and 30). The writer has often noticed these temporary venous 

 plexuses in the various stages of degeneration. In 20 mm. pig- 

 embryos, for example, such retrogressive venous channels fre- 

 quently reveal constrictions at irregular intervals along their 

 course. In later stages they begin to break up into segments, which 

 at first, however, are still connected with one another by densely 

 staining cell-strands, the remains undoubtedly of the endothelial 

 walls, and in this way indicate the pathways of the originally 

 functional vessels. The segments at the beginning possess a 

 distinct cavit}' or lumen, but subsequently the cavity becomes 

 filled up with a solid cell mass which is apparently due to a pro- 

 liferation of the former endothelial or lining cells. Such cell 

 aggregations gradually vanish in the mesenchyme perhaps by the 

 regression of their elements to undifferentiated tissue. In other 

 words, the cells which at one time functioned as the limiting walls 

 of a haemal vessel, after the elimination of such a vessel from the 

 blood channel system, lose their specialized characteristics and 

 possibly return to the mesenchyme by assuming the qualities and 

 functions of the ordinary tissue cell. By comparing the recon- 

 structions illustrated in figures 28 and 35, it is seen that the venous 

 plexuses which are so profusely developed in the 19 mm. embryo 

 have almost entirely disappeared in an embryo of somewhat 

 greater length. In the stained sections of the latter, however, 

 some of the dense cellular masses are still visible here and there in 



