THORACIC DUCT DEVELOPMENT IN THE PIG 447 



fibrils or their spur-like remnants often bridge the spaces or pro- 

 ject into them and seem to suggest the existence of stresses and 

 strains, as well as their direction, in the production of the anlagen 

 by the breaking down of barriers and the fusion of interstitial 

 spaces. 



A later stage in the transformation of the periaortic region is 

 offered by series 105, a 22 mm. embryo; but a minute account is 

 hardly necessary considering the clearness of the appended figures 

 which are self-explanatory and almost sufficient in themselves. 

 Figure 19 is from a section taken just in front of the mesonephroi 

 and is representative of the conditions active along the whole 

 range of the periaortic plexus. The vero-lymphatics (6c) have 

 lost most of their connections with the supracardinal veins and 

 throughout the greater part of their course present a shrunken 

 cavity filled with the deeply staining debris of blood cells. The 

 lymphatics (4) which enmesh them are either broad spaces and 

 have obliterated the venous core almost completely, or small 

 crevices hugging one side of a vessel which has just begun to 

 manifest degeneration. They are irregular in arrangement but 

 they may always be distinguished by their clear lumina andun- 

 specialized walls. A segment of the proximal portion of the 

 supracardinal division in a 22 mm. embryo was reconstructed, 

 a drawing of which is reproduced in figure 33, illustrating in three 

 dimensions the conditions described. 



The cisterna chyli is the outcome of a number of changes which 

 proceed in very rapid succession and at the beginning often occur 

 simultaneously: the detachment of the posterior supracardinal 

 veno-lymphatics from their venous trunks; the condensation of 

 these abandoned channels progressively toward the production 

 of a plexiform or multilocular channel; the recession of their 

 intima; the breaking down of broad partitions of tissue between 

 them; the expansion of the resultant cavity by the addition of 

 spaces from the mesenchyme; the simplification of its lumen and 

 the acquisition of a lymphatic endothelium. In other words, a 

 large part of the cavity of the cisterna chyli is derived from the 

 combined cavities of preexisting venous channels, but its wall is 

 newl} r differentiated from the mesenchyme. A consideration of 



