416 OTTO F. KAMPMEIER 



2. A transition phase, characterized by the atrophy of these 

 veno-lymphatics and the genesis of discontinuous lymphatic 

 anlagen. 



3. A lymphatic phase, in which continuity is established 

 along the whole thoracic duct anlage and secondary growth proc- 

 esses bring about its completion. 



Because these developmental changes proceed in a general 

 antero-posterior direction, more than one phase may be present 

 in the same embryo at the same time, although at different levels, 

 but so definite is the succession of events that the above division 

 into three phases will invariably suggest itself. 



1. The veno-lymphatic phase {15-19 mm. pig embryos) 



In the early embryonic history the venous plan of the thoracic 

 region is composed of two strictly symmetrical and bilateral 

 halves which are practically disconnected from each other except 

 through the heart. Later by the formation of plexuses, anasto- 

 moses and fusions, certain channels acquire more of the blood 

 current and thereby gain supremacy over others, which gradually 

 dwindle in size and vanish and consequently give rise to the start- 

 ling asymmetries of older embryonic stages and of the adult. At 

 the time when the first of these profound transformations are 

 initiated, a series of vessels are developed which function only 

 temporarily in this scheme and then disappear completely. Ref- 

 erence is here made to the 'veno-lymphatics' which have their 

 origin and consummation in those stages approximately between 

 15 and 19 mm. in length and are a part or product of the supra- 

 cardinal or azygos system during its early transformations. 



For want of a better descriptive term, the word 'veno-lymphatics' has 

 been extensively used throughout this paper but nevertheless with some 

 reluctance. A veno-lymphatic, as defined by Huntington and McClure 

 in their work on the development of the jugular lymph sac in the cat, 

 is a venous derivative which by confluence with other such channels is 

 directly transformed into the lymphatic structure. Instead of restrict- 

 ing himself to this original meaning, the writer has employed this term 

 in a somewhat different and a wider sense, as designating those tem- 

 porary embryonic venous channels which occupy topographically the 

 position of the future thoracic ducts, or other lymph ducts, but atrophy 

 and disappear during the genetic period of these lymphatics. If this 



