Florence E. Sabin 373 



divide into five main branches and two mnch smaller ones. Consequently 

 there are five definite primordial follicles, and two small ones. Both of 

 the small ones and two of the large ones show in the section. At this 

 stage there is no definite capsule, the limits of the node being determined 

 by the lympliatic vessels. The nodes increase in size by invading the sur- 

 rounding tissue, for example, the artery which here branches without the 

 node is subsequently included in the gland. This stage marks several 

 important changes. Tlie first has already been noted as being the 

 division of the artery and the corresponding multiplication of the fol- 



FiG. 12. Diagram of the primary lymph node in an embryo pig, 13 cm. long. 

 X about 33. The section shows two large follicles and two small ones. A, 

 artery; aid, afferent lymph ducts; bf, beginning follicle; ehl, efferent lymph 

 duct; f, follicle; gc, germ center. 



licles. The vein, which is not shown in the diagram, runs parallel to 

 each branch of the artery up to the point where the artery enters the 

 follicle. On entering the follicle artery and vein separate and both break 

 up into plexuses. At this stage the artery without the follicle can be 

 distinguished by a thickening of the connective tissue around, there being 

 no media, and by its smaller caliber. The vein has only a lining of 

 endothelium and there is as yet no thickening of the adventitia. Within 

 the follicle, the vessels are all capillaries, but the plexus which con- 

 nects directly with the artery is made of smaller vessels than the plexus 

 which connects with the vein. 



