PLATE 1 



EXPLANATION OF FIGURES 



1 Section of the peritoneal epithelium and the underlying mesenchyme of 

 the mesentery, in the region of the primary splenic rudiment of a 7.5-mm. pig 

 embryo. The cell at E is being crowded into the mesenchyme. 



2 Section of the peritoneal epithelium and underlying splenic tissue of a 

 15-mm. pig embryo. The two tissues are distinctly separated and show different 

 cytological features. 



3 Section through a sinus in the spleen of a 6-cm. pig embryo. Processes of 

 the mesenchyme cell, mes.c, extend into the early sinus. 



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