82 Florence E. Sabin. 



tissue cells than the surrounding mesenchyme. This thickening of 

 the mesenchyme around a plexus of lymph ducts makes the anlage 

 of a lymph node. 



A third point of interest is the spreading of the ducts from the 

 jugular sac to the skin. I want to call especial attention to the 

 great size of these ducts, one especially which leaves the lateral sur- 

 face of the sac. These ducts are the first lymphatics to reach the 

 skin ; as has been said, they first reach the skin in a human embryo 

 of about 20 mm., and by this stage they have gro-\vn over the head 

 and down over the shoulders. These peripheral vessels have not 

 been reconstructed. 



Fig. 12 shows that the sac has now been cut through by three of 

 the cervical nerves, the third, fourth and fifth. This is interesting 

 in connection with the shifting of the structures in the neck and 

 in the placing of the sacs. Just at the edge of the subclavian sac 

 is a second small beginning lymph node. This lymph node is like 

 the jugular one, consisting of bridges of thickened connective tissue 

 between a rich plexus of lymphatic capillaries. The beginning of 

 the deep lymphatics for the arm is also shown. I could not trace 

 them farther in the sections. 



The thoracic duct shows beautifully in the sections. It begins 

 at the cistema chyli as a double duct, but the right one soon crosses 

 obliquely in the plane of the coronal section to the left side and 

 joins its fellow. The duct lies adjacent to the azygos veins and 

 has many irregularities. At this stage, the duct reaches the jugular 

 sac, an advance from embryo ISTo. 6, of 24 mm., in which it only 

 extended into the thoracic cavity. 



In the angle of the bifurcation of the trachea in this embryo is 

 a clump of lymphatic vessels which possibly connect with the thoracic 

 duct, though the connection could not be made out in the sections. 

 These vessels extend a short distance along the bronchi and are the 

 first visceral lymphatics I have found in the series. 



The retroperitoneal sac is shown in Fig. 15, which corresponds 

 with the line on Fig. 12. The section shows the relation of the 



sac to the renal vein and brings out the especially large masses of the 

 sympathetic ganglia in this region. 



