DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 295 



lution the free erythrocytes pack the lymphatic channel densely, 

 SO that it often appears greatly distended. In some embryos, 

 caught at the critical and very evanescent stages, portions of 

 the plexiform or reduplicated segments of the lymphatic channel 

 are found filled with blood cells, while adjacent segments are 

 still empty. This again is the haemophoric recipient stage in 

 the development of the primitive ulnar lymphatic. As soon as 

 it is attained the distended and blood-filled lymphatic vessel 

 establishes a connection with the caudal end of the dorsal divi- 

 sion of the jugular lymphsac (fig. 14, 3). It now corresponds in 

 every respect to the veno-ljrmphatics of our earlier descriptions, 

 and hence McClure and I defined this structure in our joint papers' 

 in 1908 and 1910 as the ''primitive ulnar veno-lymphatic," in 

 the absence of any detailed knowledge of its earlier genetic stages. 



The channel next pours the blood collected in it rapidly into 

 the large reservoir of the jugular sac, through which it gains 

 access to the systemic venous circulation (fig. 15). This process 

 signalizes the haemophoric evacuating stage. As soon, almost, as 

 the evacuation is accomplished (embryos of 11 mm. to 11.5 mm.) 

 the proximal portion of the primitive ulnar lymphatic, now 

 entirely emptied, separates from the jugular sac at a point a 

 little caudal and dorsal to the course of the sixth spinal nerve 

 (fig. 16, 10). The reconstructions of this period give the im- 

 pression that the rapid increase in the size of the brachial plexus, 

 situated ventral to the lymphatic channel, is a mechanical factor 

 in interrupting the earlier connection of the lymphatic with the 

 jugular sac. 



In the following stages (12 mm. to 14 mm.) the cephalic por- 

 tion of the primitive ulnar lymphatic continues to diminish in 

 size. Its lumen becomes more and more reduced, the endothelial 

 cells originally lining the channel become enlarged, stain deeply 

 and gradually assume an irregular cuboidal form. Finally along 

 the line of the former channel there remains only a clump of these 

 deeply stained cells, reverted from the endothelial to the indif- 

 ferent mesenchymal type, to indicate the site of this segment of 

 the earlier primitive ulnar lymphatic (fig. 16, 10). In the mean- 

 while the distal portion of the primitive ulnar lymphatic, situ- 



