464 OTTO F. KAMPMEIER 



one another give rise to the larger part, perhaps, of its cavity; 

 but at the same time their endothelium recedes and degenerates, 

 and the cisterna-anlage increases in size by the addition of spaces 

 from the mesenchyme, so that, like the more anterior segments of 

 the thoracic duct anlage, it is bounded by ordinary embryonic 

 tissue cells during this early developmental period. 



3. The elongation and final continuity of the thoracic duct 

 anlage is effected by the progressive confluence of discontinuous 

 fusiform lymphatic spaces in a general centrifugal direction, prob- 

 ably determined by the impulse of the lymph flow towards the 

 radiation centers or lymph sacs. Injected specimens of the 

 early lymphatic stages certify the reality of blind uninjectible 

 anlagen beyond the farthest points to which the injecta have 

 penetrated, demonstrating that discontinuities in a developing 

 lymphatic channel are not 'appearances' found only by the study 

 of uninjected embryos. Not a shadow of evidence was discovered 

 in favor of the theory which maintains the centrifugal growth of 

 the duct by budding from the lymph sacs or the derivation of the 

 lymphatic endothelium from the veins. During the period of its 

 initial growth the thoracic duct increases in diameter by the con- 

 centric addition of enlarged and immediately surrounding tissue 

 spaces to its lumen. The intima of the thoracic duct is a differen- 

 tiation in situ of mesenchymal cells as an adaptation probably 

 to the pressure of the lymph flow within the cavity. 



