428 OTTO F. KAMPMEIER 



lumina of the replacing lymph vessels (3a, b, c, d, e), the photo- 

 graph suggests the method by which the latter enlarge, as well 

 as their mode of origin. The anlage at 3e (fig. 8) presents an 

 excellent initial stage in which the endothelium of the blood chan- 

 nel has receded from the original circumference and two small 

 mesenchymal vacuoles have appeared one on each side of the 

 points of weakness. On the other hand, the anlage at 3a, the 

 channel of the lymphatic plexus nearest to the lymph sac and 

 internal jugular vein (9), is very large and irregular and has 

 increased in size obviously by the coalescence of several closely- 

 crowded spaces, as indicated by the extremely ragged periphery 

 of its lumen and the remnants of tissue traversing it. 



The lymphatic plexus just described is widely confluent with 

 the jugular lymph sac (fig. 29) through the thoracic duct approach 

 (2). The reader will recall that this structure, the approach, is a 

 part of the sac and has its origin with it. In the preceding or 

 veno-lymphatic phase it exists normally as one (fig. 28) or two, 

 and sometimes three, short prolongations between the roots of 

 the dorsal tributaries of the jugular vein. At the time when 

 lymphatic spaces are appearing along the precardinal veno- 

 lymphatics, the sharply defined venous endothelium of the 

 approach (2, fig. 8) retracts from its former circumference, evi- 

 dently as the result of a stagnation in its growth, and becomes 

 surrounded by a clear and larger cavity (4.) which is lined with 

 ordinary unmodified mesenchymal cells, the progenitors of the 



Fig. 8 Transverse section through the left lower cervical region in a 20 mm. 

 pig embryo (series 194, slide 23, section 21), X 120. 2, thoracic duct approach, 

 and its original venous intima replaced by a large space (4) ', 3a, b, c, d, c, lymphatic 

 plexus replacing as extra-intimal spaces the precardinal veno-lymphatic plexus; 

 9, internal jugular; 13, wall of the aorta; 15, vagus; 17, oesophagus; 18, trachea. 

 (Reconstruction, fig. 32.) 



Fig. 9 An accural e camera Lucida drawing of a highly magnified area of the 

 section represented in figure 8, X 266 (reduced from X 400). 3a, l>, c, d, cross-sec- 

 tions of the extra-intimal lymphatic plexus replacing the anterior precardinal 

 veno-lymphatics; the absence of any specialized endothelium in the wall of the 

 lymphatic spaces is plainly evident; the strands of mesenchyme jutting into their 

 cavities suggest the breaking down of contiguous spaces in the formation of the 

 lymphatic plexus; 6a, the collapsed venous intima of the abandoned veno-lympha- 

 tics; 9, wall of the internal jugular; 14, sympathetic nerve trunk and branch. 



