PLATE 3 



EXPLAXATIOX OF FIGURES 



7 Section of an artery and its lymphoid sheath in the spleen of a 30-cm. pig 

 embryo, showing intermediate stages in the formation of small lymphocytes from 

 the reticulum. Cells a, b, c, d, successive steps in the line of differentiation: 

 cells e, f, g, h, free lymphoid cells very similar in nuclear structure to cells c and 

 d; I and w, large lymphocytes. 



8 Cross-section of an artery in the spleen of a 6-cm. pig. 



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