144 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



along and around the components of an atrophying embryonic 

 venous plexus, or follow part of the circumference of a more definite 

 early venous trunk. As these spaces increase, enlarge and become 

 confluent to form longer segments of the lymphatic channel, the 

 related venous element continues to recede, until, in certain areas, 

 it is entirely replaced by the lymphatic vessel. In other districts, 

 along embryonic veins which are carried into the adult organiza- 

 tion, as permanent components of the venous system, the redun- 

 dant vein of the earlier stages becomes more circumscribed and 

 more clearly defined as the companion lymphatic channel develops 

 in the perivenous area. In either case the complete or partial 

 replacement of venous by lymphatic elements does not occur 

 suddenly, but is accomplished by gradual and progressive onto- 

 genetic stages. I can find no evidence at any point of the separa- 

 tion of portions of a venous plexus or of an already organized 

 venous channel, such detached parts then uniting directly with 

 each other to form systemic lymphatic vessels. In every region 

 of the mammalian embryo the above described extraintimal 

 development of the lymphatic elements, and the subsequent 

 secondary topographical replacement of decadent veins by them, 

 can clearly be demonstrated. This process, of course, leads to a 

 stage in which the advancing development of the lymphatic and 

 the correlated regression of the associated venous components 

 are balanced, and in which both, proceeding in opposite genetic 

 directions, are found side by side in the same field. The stages 

 previously discussed in detail, between 13 mm. and 15 mm. offer 

 abundant illustration of this intermediate stage in the azygos 

 area. 



It is to be noted that in the earlie'r stages (13 to 14 mm.) the 

 lymphatic extraintimal spaces first appear on the lateral aspect 

 of the atrophying venules of the ventro-medial azygos plexus, and 

 subsequently surround them. They are then situated at some 

 distance ventral to the main azygos trunks. In the later stages 

 (15 mm., 15.5 mm. and 16 mm. embryos) they have extended 

 dorsad and lie in close apposition to the ventral and ventro-medial 

 circumference of the larger veins, which position they maintain 

 subsequently in their further development. Coincident with this 



