198 FREDERICK TILNEY 



Such evidence of lymphatic organization as this embryo affords 

 confines itself to the primitive venous fundaments of the jugu- 

 lar lymph sacs. If these elements did not so nearly duplicate 

 the conditions of the early ground plan of the sac in the cat, 

 it would be difficult, without further material, to recognize their 

 true significance. They appear as several dorsal tributaries in 

 connection with the straight portion of the pre-cardinal vein, 

 with the promontory and with the cephalic extremity of the 

 post-cardinal vein. Three of these branches are related to the 

 straight limb of the pre-cardinal, one to the promontory and 

 one to the post-cardinal The tributaries in relation with the 

 pre-cardinal vein are irregular and have the appearance of redun- 

 dancies in the vein line. One in particular, the third and most 

 caudal of the series, is larger and more irregular than the rest. 

 Another characteristic of early lymph sac formation is the marked 

 increase in size of the pre-cardinal vein and promontory as they 

 draw together toward the Cuvierian duct. This augmentation 

 does not appear to be in response to the entrance of new tributary 

 lines, but, as in the cat, seems to be a redundant growth of the 

 venous network prior to its differentiation into distinct veno- 

 lymphatic channels. Upon these grounds, and especially because 

 of their striking correspondence to the conditions observed in 

 the cat, these structures may fairly be taken to represent the 

 elements which determine the early or primary venous stage of 

 lymph sac organization. 



The venous organization of the 6 mm. embryo (fig. 5 and fig. 6) 



The advance observed in this embryo is characterized by the 

 emergence of a definite sub-cardinal drainage line out of the 

 perimesonephroic plexus and the disturbance of the bilateral 

 symmetry in the umbilical veins. There are five sets of paired 

 venous channels, the general disposition of whose course is paral- 

 lel to the long axis of the body. The perimesonephroic and 

 umbilico-cardinal plexuses have lost their definite outlines. 



1. The omphalomesenteric veins (fig. 6, 9). Notwithstanding 

 the fact that the cephalic portion of each vitelline vein has become 



