RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 



29 



will depend on the plane of section in reference to the course of 

 the lymphatic and its contained venous remnant. Thus, as 

 shown in the following schema, many sections give the appear- 

 ance indicated in 1. If the plane of section should, however, lie 

 in the line A B, it will divide the shrinking vein (4) and the envel- 

 oping lymphatic (5) in such a way as to produce the picture shown 

 under 2. In other cases the lymphatic spaces unite around 

 the entire circumference of the abandoned venule, and the lumen 

 is then contained for long distances entirely within the lumen 

 of the replacing lymphatic channel. 



The process just described is remarkably constant and uniform 

 in the critical stages of mammalian lymphatic development. 

 As can be readily seen in following the individual sections in 

 the microphotographs published in Part II, the significance of 

 the conditions here shown is unmistakable. This is not a hap- 

 hazard process, observed only occasionally, in a limited number 

 of embryos, and then only in single sections, or, at most, in a few 

 successive sections. In any average embryo of the proper age 

 the same structures appear regularly in the same situations and 

 in identical relationship to the embryonic environment. It is 

 often possible, as the microphotographs and the corresponding 



