Florence 11. Sabin 371 



The point of entry of the artery determines the hihim of the node. The 

 position of tlie hiiuni is determined from the lieginning of the formation 

 of the node hy the lines of growth. Hh^od-vessels and lymphatics grow 

 from the center of the body to the periphery, so that the proximal surface 

 of the gland has from the start the entering blood-vessels and the efferent 

 lymphatic ducts, while the peripheral surface of the node is the place 

 from which the efferent lymphatic ducts radiate to the area they are to 

 drain. In the central core of connective tissue the lymphatic capillaries 

 are reduced in number and size; they are never quite absent but do not 

 appear except in well-injected specimens. The presence of these ducts 

 within the connective tissue core may have some liearing on the pathology 

 of lymph nodes. The disappearance of the lymph capillaries in the 

 center of the node involves the retrogression and absorption which is 

 characteristic of developing tissues. Throughout the evolution of the 

 lymph node there is continnal building up and tearing down. This will 

 be evident in later stages where there is a continual change in the pro- 

 portion of the lymphoid structures or cords and the lymphatic struc- 

 tures or sinuses. 



Beside being the stage which marks the beginning of the adult struc- 

 tures of the node, that is to say, of the follicle, this stage also shows 

 fundamental changes in cell differentiation. It marks the beginning of 

 the wandering cell in lymph nodes. Up to this time the connective 

 tissue part of the node has consisted of a network of granular proto- 

 plasm with many nuclei, young, dividing, and old. At this time three 

 types of wandering cells appear, the lymphocyte, the polymorphonuclear 

 form, and the eosinophile. 



Lymphocytes are present in the thymus at a much earlier stage, they 

 are abundant there in the sections from the enil)ryo 3.6 cm. long. In 

 the sections of the lymph node at 8 cm., there are a few lymphocytes in 

 the connective tissue core of the node, and in little clumps in the con- 

 nective tissue just without the node. These little clumps of cells are 

 found near the capillaries. The differences between the connective tissue 

 cell and the lymphocyte are as follows: The nucleus of the former is 

 large, faintly staining, and oval in shape, and the protoplasm belongs 

 definitely to the network, while the latter has a small, round, deeply 

 staining nucleus, with a more distinct nuclear membrane. The nuclear 

 network and the chromatin granules are coarser, and there are one or 

 more nucleoli. Moreover, the protoplasm makes a narrow but definite rim 

 around the nucleus. Between the connective tissue cell, especially the 

 _young forms, and the lymphocyte one can see every possible transition. 



