DEVELOPMENT OF THE MAMMALIAN SPLEEN 311 



ure 8 shows two darkly staining nuclei near the arterial wall, but 

 cells of the same type are encountered at various distances from 

 the vessel, so that no particular significance can be placed upon 

 their differentiation at this stage. 



However, in a 7.5-cm. embryo a marked differentiation has 

 taken place. Figure 6 is a section of an early arterial capillary 

 that is cut somewhat obliquely. The endothelium lining the 

 vessel possesses large clear nuclei, oval in outline, that extend so 

 far into the lumen of the vessel that only a narrow slit-like open- 

 ing is discernible. Surrounding the endothelium is a group of 

 small reticular cells that closely resemble the forms seen in a 30- 

 cm. embryo (fig. 7, cell a). At this stage (7.5) most of the small 

 cells have open nuclei — none being present that have differenti- 

 ated into true small lymphocytes. 



By comparing the cells around the artery with those of the 

 surrounding reticulum, one is soon convinced that both are parts 

 of a general splenic mesenchyme that has begun to differentiate 

 along two quite distinct lines. 1) That immediately around the 

 arterial wall has drawn in its processes until the cells lie close 

 together and are connected by short delicate cytoplasmic strands. 

 Many of the nuclei are small, round, deeply staining structures, 

 with a distinct nuclear membrane and one or two coarse chro- 

 matin granules. 2) The mesenchyme surrounding the more 

 compact area has become converted into a loose spongy network, 

 the meshes of which are crowded with fully differentiated eryth- 

 rocytes, which have, without doubt, been brought in by the 

 blood-stream, as there are but few isolated regions of erythropoi- 

 etic activity in the spleen at this stage. By a careful study of 

 the two areas, all possible stages between the cells around the 

 arterial wall and those of the general reticulum can be identified. 

 The above findings are closely related to the observations on 

 the development of the lymph nodes in the pig as reported by 

 Sabin. The latter found the first evidence of the formation of 

 Ivmphatic nodes in embryos 3 cm. long. In these early nodes the 

 capillaries penetrating the connective tissue are surrounded by 

 clumps of nuclei which lie 'Svithin the syncytium and belong to 

 the connective tissue." They are distinguished from those of 



