DEVELOPMENT OF THE THORACIC DUCT 105 



The only conclusion which seems to me to be warranted by 

 actual observation is that certain embryonic veins form, during 

 the process of their atrophy and final elimination from the definite 

 venous organization, the supporting lines along which certain 

 of the perivenous extraintimal lymphatic anlages first develop. 



The fact that numerous early embryonic venous channels, 

 large and small, atrophy and disappear during the normal course 

 of subsequent development, appears to afford a more favorable 

 field for the greater development of the adjacent mesenchymal in- 

 intercellular spaces, so that these enlarge more rapidly, as the corre- 

 lated vein recedes. This relationship appears, however, to be 

 based exclusively on the physical and mechanical ad vantages which 

 the abandoned and shrinking primary venous line affords to the 

 adjacent mesenchymal spaces for more rapid enlargement, in 

 the sense of replacing the disappearing vein and occupying second- 

 arily the space formerly filled by the haemal channel. This is 

 evidently an important factor in determining the size and extent 

 of the final lymphatic channel resulting from the confluence of 

 the originally separate and independent perivenous anlages. 

 Consequently, in the adult, the largest and best defined systemic 

 lymphatic vessels either accompany reduced adult remnants of a 

 relatively larger embryonic venous channel, or, in case of the 

 latter's entire default, topographically replace the same. Now, 

 while this relation manifests itself strikingly in many parts of 

 the body, it is quite evident that the development of lymphatic 

 channels occurs in other parts independently of preceding veins, 

 by the confluence of independent intercellular mesenchymal 

 spaces. 



The cat may, and does, differ in its details of lymphatic develop- 

 ment, and in its adult lymphatic organization, from the condi- 

 tions obtaining in many other mammalian forms. In fact, even 

 in the present very incomplete state of our knowledge of the com- 

 parative anatomy of the mammalian lymphatic system, we 

 realize that what we regard as the mammalian ground plan of 

 lymphatic organization may in certain genera undergo great and 

 far-reaching modifications, as for example the conditions deter- 



