312 GEO. A. THIEL AND HAL DOWNEY 



the remaining portion of the connective tissue by the fact that 

 they are situated in clumps and that some show mitotic figures, 

 while others are smaller and take the deep stain of a newly divided 

 nucleus. Up to an S-cm. pig, the node has none of the charac- 

 teristics of the adult structure. There are no lymph cords, nor 

 germ centers, no lymphocytes and no sinuses. In an 8-cm. em- 

 bryo the artery is seen lying parallel with the vein and extending 

 into the core of the node. Associated with the development of 

 the arterj^ is the formation of the first lymphoid elements of the 

 node. Surrounding the arterial capillaries are groups of small 

 cells that can readilj^ be separated from the adjoining connective- 

 tissue cells. The latter possess large, faintly staining and oval 

 nuclei and their protoplasm is in the form of a definite network, 

 whereas the smaller cells have deeply staining nuclei and a dis- 

 tinct nuclear membrane. The nuclear network and the chroma- 

 tin granules are coarser and there are one or more nucleoli. 

 Moreover, the protoplasm makes a narrow but definite rim 

 around the nucleus. 



Sabin reports that "between the connective tissue cells and 

 the lymphocytes one can see every possible transition." Yet the 

 presence of all intermediate stages is not held as sufficient evi- 

 dence to prove their local origin, for in some cases the clumps of 

 free cells appeared as though they had filtered through the ves- 

 sel wall. Sabin is of the opinion that such evidence is not suf- 

 ficient to prove either the hematogenous or connective-tissue 

 origin of the lymphocytes. 



The group of small cells around the capillaries in the nodes, 

 as described by Sabin, remind one of the small mesenchyme cells 

 surrounding the early arteries of the spleen. Sabin states that 

 in early stages the small cells are distinctly a part of the connec- 

 tive tissue. The same is true in the spleen, although many cells 

 can be found in the process of rounding off to form free lymphoid 

 cells. 



Danchakoff sees the arteries grow into the chick spleen at about 

 the middle of embryonic life. The mesenchyme around the 

 arteries proliferates and forms lymphoid hemocytoblasts by the 

 rounding-off and isolation of its cells, but their differentiation 



