DEVELOPMENT OF THE THORACIC DUCT 83 



2. THE PREAZYGOS SEGMENT 



This portion of the thoracic duct develops, again by confluence 

 of numerous extraintimal perivenous anlages, in the cephalic 

 and lateral mediastinal space, between the termination of the 

 thoracic duct approach of the jugular sac and the level of the 

 intersection of the left duct with the aortic arch. Its ontogenesis 

 offers the most striking and definite evidence of the development 

 of the systemic lymphatic channels in this mammalian embryo 

 as the result of the confluence of a large number of originally 

 separate and independent "extra-intimal" or "perivenous" 

 mesodermal spaces, following and enveloping the early embryonal 

 veins, or completely surrounding them, and finally entirely replac- 

 ing them. The spaces which I am describing as the anlages of 

 the systemic lymphatic channels in the mammalian embryo under 

 consideration (Felis domestica) are surely easy enough to see. If 

 they are followed in successive stages they fully and completely 

 reveal the histogenesis of the systemic lymphatic vessels in this 

 mammalian form, and conclusively establish the relation of the 

 first mammalian lymphatic anlages to the embryonic veins, which 

 they are destined to replace, as previously outlined in the prelim- 

 inary publications quoted (1, 2, 21, 22). The structures which 

 form the first anlages of the systemic lymphatic channels in 

 the embryo of the cat cannot be injected, because at the time 

 of their first definite appearance they represent the still disjointed 

 links of a chain which is only subsequently to be assembled into 

 a continuous whole. It is possible to inject in earlier periods 

 the veins which are later replaced by these extraintimal lymphat- 

 ics, and it is possible to inject in later stages the lymphatics 

 when they have united into a connected system of channels. 

 But in either case injection methods merely prove topographi- 



