418 OTTO F. KAMPMEIER 



which proceed caudally as two large and important vessels and 

 only very much later take a further part in the production of the 

 asymmetrical venous plan of the trunk, especially in their trans- 

 formation with the postcardinals to form the azygos system of 

 veins; this segment may therefore be termed the supracardinal 

 division. Since the pre- and postcardinal divisions (A, B) of the 

 longitudinal supracardinal lines disappear or lose their independ- 

 ence, either by being transformed into transient venous plexuses 

 as in the first division or by fusing entirely (12/rf, 12ls) with the 

 postcardinals as in the second division, it is evident that the dorsal 

 segmental tributaries (8), which in the beginning spring from 

 these lines, must shift their roots so that they arise directly from 

 the pre- and postcardinal veins in the two divisions mentioned. 

 In the third or posterior division (C), however, these segmental 

 tributaries (8) continue to return their blood to the supracardinals, 

 for the latter exist as independent venous trunks. 



Because the three divisions in the transformations of the supra- 

 cardinal lines coincide perfectly with, or better map out, three 

 well-defined regions in which the history of the thoracic duct is 

 enacted, the terms, pre-, post-, and supracardinal divisions are fully 

 as applicable here, the events in this history occurring immedi- 

 ately along the pre-, post-, and supracardinal veins, respectively. 



(A) Precardinal division. In 17 to 19 mm. embryos the 

 anterior segment of the left supracardinal line, originally a simple 

 longitudinal channel as in the 15 mm. embryos, has been trans- 

 formed into an intricate plexus the roots of which now appear as 

 numerous dorsal tributaries of the internal and common jugular 

 and precardinal veins. The branches of these tributaries extend 

 dorsad and vascularize the areas on both sides of the sympathetic 

 nerve trunk, but in number and complexity the internal branches 

 exceed the external ones. As will become evident later, an im- 

 portant distinction is potentially present between these two sets 

 of branches, and, although the lack of differentiation at this 

 stage would not warrant the use of specific terms, in the light 

 of future events they may be described as precardinal veno- 

 lymphatics, lying internal and mesial (6a, figs. 1 and 28), and 

 precardinal segmental veins (8), functionally related to the ter- 



