DE^^ELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 293 



ascends as the primitive ulnar vein (5) , arches over the sixth spinal 

 nerve (9) and enters the dorsal aspect of the precardinal vein (1) 

 (jugular promontory), in close relation to the caudal end of 

 the jugular lymphsac (dorsal division, 3, fig. 12). Along the 

 dorso-medial circumference of this proximal segment of the prim- 

 itive ulnar vein a series of intercellular clefts develop in the 

 mesenchyme (5). These clefts enlarge rapidly, unite with each 

 other and form a channel, the primitive ulnar Ijonphatic, which 

 parallels the vein of the same name (8) (fig. 13). At this 

 time for a short period the primitive ulnar lymphatic contains 

 no blood cells, but is distended with clear fluid and lined by a 

 layer of flattened cells which are assuming endothelial character. 

 The blind cephalic end of the lymphatic channel extends to 

 within a short distance of the caudal extremity of the jugular 

 lymphsac, but is "as yet not connected with the same. In the 

 mesenchyme surrounding the primitive ulnar lymphatic along 

 its entire course are numerous small collections of blood cells in 

 all stages of development (7, figs. 12 and 13). 



The primitive ulnar lymphatic appears in this condition in 

 cat embryos between 8 mm. and 9 mm. and even up to the 9.5 

 mm. stage. During this period the channel still reveals its 

 formation through fusion of a large number of separate and 

 originally independent collections of intercellular mesenchymal 

 spaces. This accounts for the occurrence at intervals in the 

 course of the main channel of multilocular and plexiform areas. 

 In these areas the character of the cells limiting these primitive 

 spaces ranges from the typical indifferent mesenchymal cell to 

 a complete and distinct endothelium. The mode of development 

 of the primitive ulnar lymphatic also accounts for the redupli- 

 cation of the channel frequently observed, two parallel canals 

 running side by side for longer or shorter distances. 



In the succeeding stage (10 mm. to 10.5 mm.; fig. 14) the 

 primitive ulnar lymphatic (10) becomes filled with red blood 

 cells derived from the clumps of developing blood-islands which 

 lie in the mesenchyme in close propinquity to the Pymphatic 

 channel. The lumen of the latter extends to include and sur- 

 round these areas of developing blood cells. After their reso- 



