104 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



Consequently, in the pictures of the earlier stages (13, 13.5 and 

 14 mm. embryos) the area occupied by the still capacious aban- 

 doned venous channel, combined with that of the relatively narrow 

 enveloping lymphatic space, appears larger and more prominent 

 than in the later periods (15, 15.5 mm.) in which the permanent 

 lymphatic channel and its reduced venous kernel occupies less 

 histological territory. 



It should also be noted that in the earlier stages (13, 13.5, 

 14 mm.) the perivenous extraintimal lymphatic network is more 

 diffuse and redundant than in the later stages (15, 15.5, 16 nmi.). 

 This early plexiform lymphatic reticulum is subsequently con- 

 densed into more limited and better defined lymph channels, in 

 exact conformation to the processes observed in the haemal 

 vascular development, in which the definite blood channels 

 crystallize along main lines out of an antecedent indefinite and 

 more diffuse plexus. The result of this genetic process is strik- 

 ing in tracing developing lymphatics through successive stages. 

 Thus in the region here under discussion the pictures offered by 

 the 13, 13.5 and 14 mm. embryos are more conclusive than those 

 furnished by the 15 and 15.5 mm. stages in reference to the gene- 

 sis of the lymphatic channels. In the former, the lymphatic 

 anlages and the decadent venules around which they develop are 

 taken together, relatively larger and more numerous, and hence 

 easier to recognize. In the latter, this early redundant perive- 

 nous lymphatic plexus has been replaced by a relatively smaller, 

 but much more definite and distinct channel, occupying, however, 

 always a part of the area filled in the earlier stages by the less 

 circumscribed antecedent lymphatic plexus with its contained 

 remnants of abandoned embryonic veins. I lay special stress on 

 this fact, because it is absolutely necessary to take it into account 

 in comparing stages between 13.5 and 15.5 mm. In the latter 

 stage the final lymphatic anlages have definitely formed and have 

 reached the relation to adjacent main venous lines which they 

 henceforth are to occupy. The further growth and enlargement 

 of these channels begins in the 16 mm. embryo, and proceeds 

 from this point on, in many regions at the expense of the adjacent 

 embryonic vein undergoing atrophy. 



