PLATE 2 



EXPLANATION OF FIGURES 



2 to 8 show changes in the germinative wall and the size of the tubule in rats 

 from 7 to 14 days after birth. All X 800. 



9 to 14 show steps in the transformation of the undifferentiated cells to the 

 pachytene stage of development. All X 1200. 



Abbreviations: D.C., dividing cell; A. B.C., cells of certain types. 



2 Oblique section of tubule from 7-day testis. Section 10 micra thick; 

 stained with iron-haematoxylin and orange G. Germinative cells all alike (type 

 A). Two dividing cells (D.C.) appear, one out of focus. 



3 Transverse section of tubule from 11-day testis. Section 2 micra thick; 

 stained with iron-haematoxylin and acid fuchsin. Differentiation has begun, as 

 shown in cell type B. 



4 Section of tubule from 12-day testis. Section 2 micra thick; stained with 

 iron-haematoxylin and acid fuchsin. Differentiation has progressed to cell type 

 C. 



5 Portion of transverse section of tubule from 14-day testis. Section 5 micra 

 thick; stained with iron-haematoxylin.- Shows dividing cells (D.C.) of type A 

 organizing into cells of type B; also three cells in pachytene stage. 



6, 7 and 8 from the same testis as figure 5; thickness and stain also the same. 

 Figure 6 is from a slightly oblique section ; figure 8 from one as nearly transverse 

 as could be found. The outer row of cells in figure 6 is chiefly of type B, the result 

 of the process taking place in figure 5 ; the next row passing toward the lumen shows 

 cells chiefly of type A ; three cells in the pachytene stage are seen as in figure 5. 

 In figure 7, upper portion, cells of type C have replaced those of type B, figure 6, 

 and have crowded inward to lie between cells of type A. Compare figures 9 and 

 10. The lower tubule in figure 7 and figure 8 show progressively later stages. 



9 to 12 are from 15-day testis; 13 and 14 from 16-day. All from sections 5 

 micra thick and stained with iron-haematoxylin. 



9 and 10 show details of types B and C; compare with figures 3 to 7. 



11, 12 and 13 show the progressive development of the spireme up to its lepto- 

 tene stage. Compare with figure 7, lower tubule, and figure 8. 



14 illustrates the pachytene, or paired thread stage. 



15 is a sketch made from the same section as figure 14; magnification not taken. 



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