Florence R. Sabin 375 



cells. The reticulum fibers develop subsequently in the connective tissue 

 bridges (not. as Kling says, from the endothelium). Tlie lymph ducts 

 from which the sinus is made have a complete endotliolial lining. More- 

 over, the increase in the ducts which form the sinus takes place by the 

 same process of the budding of endothelial cells which characterizes the 

 development of lymphatics or blood-vessels elsewhere. Thus the spaces 

 of the sinus are lined throughout by endothelium. The sinus can be pic- 

 tured in three dimensions by imagining the follicle surrounded by a plexus 

 of ducts so dense that the bridges between them are reduced to the thick- 

 ness of a single network of fibers. Such a structure in cross section would 

 give the appearance of fibers with endothelial cells around them cutting 

 the lumen of the sinus. As a matter of fact the fibers are between the 

 endothelial cells and without the lymph channels. They connect with 

 the rest of the connective tissue framework of the node as is readily seen 

 in Fig. 12. 



The next stage is from a section of the primary lymph node of an 

 embryo 15 cm. long (Fig. 13). There is now a great development of 

 the artery. The wall of the artery has developed considerably and shows 

 one row of smooth muscle cells beside the adventitia. The vein which lies 

 beside it has only an endothelium and a thickened connective tissue sheath. 

 The artery divides into three main branches on the edge of the node, and 

 within the node each branch subdivides several times. As in the early 

 stage, the artery and vein run parallel until the follicle is entered and 

 there they separate. The amount of lymphoid tissue has increased par- 

 allel with the development of the artery. The sinuses are growing down 

 into the node between the arteries, thereby surrounding and limiting the 

 lymphoid masses around the blood-vessels. By this process the lymph cord 

 along the blood-vessel becomes defined, as will be clear in the next dia- 

 gram. 



The especial advance in this stage lies in the connective tissue, for here 

 the reticulum fibers within the node first begin. Up to this stage the 

 connective tissue of the node has been a protoplasmic network with delicate 

 anastomosing fibrils which, however, do not stain sharply by Mallory's 

 method. Xow for the first time there are a few fibrils which stand out 

 clearly in the Mallory stain. They occur in the germ centers where they 

 are laid down in concentric circles. With the oil immersion lens it can 

 be seen that the fibers of the germ centers make a definite mosaic of poly- 

 gonal spaces in concentric rows. All of these polygonal spaces thus out- 

 lined are filled with cells. This appearance of the mosaic can be seen 

 in thin sections of the adult node and can be brought out by silver nitrate. 



