PLATE 2 



EXPLANATION OF FIGURES 



4 Section of a venous sinus in the spleen of a 24-mm. pig embryo. Some of 

 the mesenchyme cells lining the sinus are elongated into typical reticulo-endothe- 

 lial structures. The cell {endo.c.) on the right wall shows strongly basophilic 

 cytoplasm and a nucleus similar to that of a large lymphocyte. The cell is being 

 cut off from the mesenchyme. 



5 A group of hemocytoblasts in the mesenchyme of the spleen of a 4-cm. 

 pig embryo. The structure of the nuclei of cells A and B is intermediate between 

 that of the mesenchyme cell, mes.c, and the hemocytoblast C. 



6 A slightly oblique section of an artery and surrounding mesenchyme in 

 the spleen of a 7.5-cm. pig embryo. The presence of numerous small and medium- 

 sized dark nuclei indicates the beginning of the differentiation of small and 

 medium-sized lymphocytes from the mesenchyme. 



336 



