RELATION OF LYMPHATIC TO BLOOD- VASCULAR SYSTEM 27 



In the succeeding stages the mesodermal lymphatic anlages 

 assume, in large part, the very definite relation to certain embry- 

 onic venous channels, which led McClure and myself to describe 

 them in 1906 in our preliminary account above quoted, 21 as 

 " extraintimal" or "perivenous" structures. 



This relationship is of two kinds: 



A. Total replacement of temporary embryonic veins by extrainti- 

 mal lymphatic channels. 



In the earlier embryonic stages the areas of the future definite 

 venous channels are largely occupied by an antecedent venous 

 or capillary network, out of which, along definite hydrostatic 

 lines, the subsequent veins develop by confluence of the plexus ele- 

 ments occupying these lines. 29 Parts of the early capillary reticu- 

 lum, not thus included in the path of the definitely organized 

 venous trunks, remain, after the latter have become established, 

 as a perivenous plexus. Some of the elements of this secondarily 

 established plexus develop into permanent tributaries of the 

 main veins. Others undergo a process of separation from the 

 permanent functional channels and degenerate. In many cases 

 their blood-cell contents break down and are eliminated, while 

 their endothelial lining appears to revert to the indifferent type 

 of the embryonic mesodermal cell. 



Thus in embryos between 13.5 and 16 mm. many striking 

 instances of this reversion are to be observed. The former 

 vascular channel appears as a collection of clearly differentiated 

 and very highly stained mesodermal cells. 



Figs. 47, 50, 51, in Part II, show these mesodermal vascular 

 derivatives very clearly. They form the dark masses seen in 

 the field dorsal and dorsolateral to the oesophagus and in the 

 peritracheal region. 



In many regions of the mammalian embryo, however, these 

 detached and retrograding venous elements do not attain this 

 condition, but in an earlier stage, constitute lines around which the 

 most active primary lymphatic organization of the mammalian 



*' H. v. W. Schulte and Fred. Tilney : "Note on the Organization of the Venous 

 Return, with Especial Reference to the Iliac Veins." Anat. Record, vol. iii, no. 

 11, 1909. 



