REFERENCES ON PLATES 



A. Ere, amitosis of erythroblasts 



hl.v., blood vessel 



D.d.e.N., debris of degenerated epithe- 

 lial nuclei 



D.e.N., degenerating epithelial nucelus 



D.e.N'., completely degenerated epi- 

 thelial nucleus 



E.N., epithelial nucleus 



Eo.C, eosinophile cells 



Eo.G., free eosinophile granules 



Erb., erythroblast 



Ere, erythrocyte 



L.L., large lymphocyte 



L M., limiting membrane 



M.C., mesenchymal ceil 



M.e.N., mitosis of epithelial nuclei 



Me.L., medium sized l>inphocyte 



M.L., mitosis of lymphocyte 



Nmb., normoblast 



Pc, phagocyte 



S.i., interlobular septa 



S.L., small lymphocyte 



T., trail 



v., vacuole 



.\., cell of unknown origin and sig- 

 nificance 



Z.pr., zone of rapid i>rolifpration of 

 epithelial cells 



PLATE 1 



EXPLANATION OP FIGURES 



1 Camera lucida drawing of a portion of a lobule of a section of the right thy- 

 mus head in a 30 mm. embryo. The infiltration of the thymus by extrathymic 

 lymphocytes from the surrounding mesenchyme has just begun. In order to 

 reduce the size of the drawing the very large lymphocyte represented in the 

 outer border of the mesenchyme was drawn a little nearer the thymus than it 

 really is. L5 mm. Zeiss App. objective, X 12 Comp. ocular; reduced one-half. 



2 Camera lucida drawing of a portion of a lobule of a section of the left thor- 

 acic segment of the thymus in a 37 mm. embryo. The thymus in this develop- 

 mental stage is slightly more advanced in development than in the 30 mm. 

 embryo. This drawing was made to show particularly the large number of 

 epithelial nuclei that are in the first stages of degeneration and the trail that was 

 apparently made by an active lymphocyte that migrated into the thymus from the 

 mesenchyme surrounding it. 1.5 mm. Zeiss App. objective, X 12 Comp. ocular; 

 reduced one-half. 



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