44 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



with the same. Nowhere is there any suggestion of a bud or an 

 outgrowth from the vein as forming the origin of these lymphatic 

 spaces. It now remains to clearly prove the genesis of these 

 spaces, and to trace their growth from their inception up to the 

 stages just pictured in which fully organized lymphatic and ve- 

 nous channels lie side by side in the mutual relation above fig- 

 ured and described. The proof of their origin is furnished by the 

 series of microphotographs of successive sections of the earlier 

 stages given in Part II of this communication, in connection with 

 the individual series described and figured in tracing the develop- 

 ment of the preazygos and azygos portions of the thoracic ducts. 

 The microphotographs, and especially the reduced reproductions 

 figured, are not so clear as the actual preparations, because focal 

 adjustment is required to follow the endothelial lining of the 

 spaces in their entire circumference, and because they lack the 

 differential stain of the sections. Still they are sufficiently dis- 

 tinct to establish definite conclusions. Merely referring, there- 

 fore, at this time to the following detailed illustrations, the general 

 topic of extraintimal replacement of embryonic veins by lym- 

 phatic spaces and the character of the latter deserve some further 

 consideration. 



The lymphatic anlages, as above stated, if studied under suffi- 

 ciently high power and with some care, are seen to begin as inter- 

 cellular clefts in the periaortic mesoderm, adjacent to the postcar- 

 dinal venous plexus, and chiefly on its ental aspect, between it and 

 the aorta. 



The individual lymphatic spaces, at first small and separated 

 from each other, enlarge, elongate and become confluent, to form 

 larger continuous channel segments, while innumerable newly 

 formed spaces of the same character appear in the surrounding 

 tissue, join with each other, and with the earlier preformed lym- 

 phatic channels, in exactly the same manner, and with the same 

 appearance of lymph endothelial "budding" or " sprouting" as 

 is observed in haemal vascularization of new areas by the junction 

 of the earlier blood capillary anlages with secondary haemal 

 plexuses. In these later stages the veins are surrounded by a close 

 lymphatic plexus, which, however, does not as yet form a con- 



