DEVELOPMENT OF THE THORACIC DUCT 111 



shrinking venule, or they occasionally completely surround the 

 same. In the former case the contained venous remnant is 

 connected at part of its periphery with the wall of the envel- 

 oping lymphatic space. In the latter and more exceptional case, 

 the free ventral nucleus of the lymphatic space is formed by the 

 atrophied vein. In either case the lumen of the latter may con- 

 tain a few degenerated red blood cells. These pictures are again 

 constant in embryos of the appropriate stages. The lymphatic 

 anlage can be accurately traced from its indefinite beginning 

 among the perivenous mesenchymal intercellular clefts through 

 a number of successive sections to its similar distal terminations 

 in the same intercellular plexus. Following the sections from 

 this point caudad through a varying intervening area in which 

 no distinct lymphatic channel appears, the same line will sooner 

 or later reveal the repetition of the same process, and the formation 

 of another link in the still disjointed chain of primitive lymphatic 

 anlages. 



It is again to be noted here that in the earlier stages (13, 13.5, 



14 mm.), the individual spaces, plus their kernel of atrophied 

 vein, are relatively larger than in the later (15-15.5 mm.) em- 

 bryos. In the earlier stages (13 mm., 13.5 mm. and 14 mm. 

 embryos) these separate and still isolated segments of the future 

 continuous lymphatic channel begin and end blindly. In the 



15 and 15.5 mm. stages they have increased in number and ad- 

 jacent links have united to form longer segments. Finally, in 

 the 16 mm. cat embryo, the preazygos portion of the thoracic 

 duct is usually established as a nearly continuous channel through 

 the further confluence of the originally separate and distinct 

 components. Every stage of the extraintimal lymphatic develop- 

 ment can be observed with absolute certainty and accuracy in 

 every series within the proper length limits. 



