28 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



embryo centers. The intercellular mesodermal clefts above 

 described develop especially along and around these decadent 

 venules and finally envelop them. As the result of this process the 

 lymphatic anlages appear in certain mammalian ontogenetic 

 stages, in large part, as distinct spaces enclosing the remnant of 

 the embryonic vein. The latter may still, for a time, contain a 

 few degenerating red blood cells, but these soon disappear, and 

 then the entire anlage is formed by a collapsed and empty en- 

 dothelial tube, the abandoned channel of the earlier vein, sur- 

 rounded by a second endothelial tube, formed by the confluence 

 of the independently developed extraintimal or peri venous meso- 

 dermal spaces. As these spaces enlarge and join each other their 

 lumen increases, and the limiting cells become flattened and 

 assume typical endothelial characters. The height of this phase 

 of lymphatic development is reached in embryos of the cat in the 

 14 mm. stage, and numerous demonstrations of the appearance of 

 the structures on section are given in Part II of this paper. The 

 remnant of the embryonic vein bears a relation to the replacing 

 perivenous lymphatic channel which is exactly the same as that 

 of a collapsed inner tube to the enveloping shoe of a pneumatic 

 tire. The inner skin of the shoe and the rim of the wheel repre- 

 sent the lymphatic intimal endothelium. The space between the 

 shoe and the collapsed inner tube is the lumen of the future 

 lymphatic channel. The empty inner tube itself is the decadent 

 embryonic vein upon and around which the secondary lymphatic 

 channel is built. In the course of further development the venous 

 remnant disintegrates and disappears, leaving a clear lumen for 

 the lymphatic vessel, which thus completely replaces the earlier 

 vein and comes to occupy absolutely the topographical position 

 of the latter. 



Often the replacing lymphatic begins as an extraintimal 

 channel partially surrounding the receding vein. This leads 

 in course of further development to an expansion of the lymphatic 

 channel not concentric with the axial line of the shrinking vein. 

 The remnant of the vein then retires to a point on the intimal 

 surface of the new lymphatic channel, and appears to project 

 into the lumen of the latter. The resulting histological picture 



