VEINS AND LYMPHATICS IN TRAGULUS 203 



the tributaries coming into the promontory and cephalic portion 

 of the post-cardinal have joined to form a large blood space. 

 The tendency of the pre-cardinal and post-cardinal veins to 

 increase in size as they approach the duct of Cuvier is still evi- 

 dent. On the right side there are clear signs of a beginning 

 fenestration in the base of the pre-cardinal, thus indicating the 

 inception of the para-pre-cardinal line. 



Marked changes are met with in passing from the primitive 

 organization of the 6 mm. embryo to the condition of the 20 mm. 

 embryo. As touching upon the stages intermediate between 

 the two, reference will be made to the 13 mm. specimen. A 

 noteworthy acquisition in the 20 mm. embryo is found in the 

 now fully developed lymph sacs, and also in the formation of 

 the main segments of the systemic lymph channel. These two 

 elements are as yet entirely distinct and separate. The plan 

 of venous drainage foreshadowed in the 6 mm. embryo has been 

 carried well on towards completion. In this process a single 

 post-cava has been acquired as far as the inter-renal anastomosis 

 while below that level, two large symmetrical channels, repre- 

 senting the cardinal collateral veins, cooperate in the formation 

 of a double post-cava. 



The lymphatic organization in the 20 mm. embryo (jig. 7) 



In the 20 mm. embryo, the jugular lymph sacs (fig. 7, 25) 

 are situated in the neck region, one on either side, on the lateral 

 aspect of the great vessels and nerves. Each sac has a general 

 wedge shape, with its base looking inward and forward, while 

 its edge lies between the vertebral column and the dorso-lateral 

 surface of the body. Its greatest diameters are attained about 

 midway between its cephalic and caudal poles. The 3rd, 4th, 

 5"th and 6th cervical ganglia lie dorsal to the sac. The third 

 cervical nerve traverses its cephalic pole, the sixth nerve passes 

 beneath its caudal pole, while the fourth and fifth nerves go . 

 directly through it. The right sac is 1.61 mm. in length, the 

 left sac 1.73 mm. The maximum ventro-dorsal diameter of 

 the right sac is 1.68 mm., that of the left sac being 1.6 mm. The 



