John P. ]\runson 341 



EXPLANATION OF PLATES. 



PLATE L 



Fig. 1. Longitudinal section of a germinal ridge of the ovary of the 

 tortoise, showing the stroma cells, nuclei of oocytes and caryokinesis of 

 oogonia; also varying stages of growing oocytes with the beginnings of 

 nucleoli in the germinal vesicle; the centrosome and Its transformation into 

 the cytocenter of the large eggs; also the variation in direction of the egg 

 axis. The largest egg is a section at right angles to the egg axis showing the 

 cytocenter, and the yolk-nuclei in the cytoccel. In the next largest egg are 

 also seen various larger forms of the yolk-nucleus. Here also is seen the 

 relation of the egg to the two outer epithelial tunics, in the outer oi which a 

 blood-vessel is seen. 



Fig. 2. Transverse section of germinal ridge near the proximal end, show- 

 ing various stages of tbe egg with its follicle, and its tunics as in Figs. 1, with 

 the centrosome and sphere variously developed, and yolk-nuclei scattered 

 throughout the cytoplasm of one of the large eggs. This shows also the 

 variation in the egg axis. 



Fig. 3. Horizontal section of the germinal ridge showing oogonia of 

 various sizes, their final division by caryokinesis, and the formation of 

 follicle and oocytes. 



Fig. 4. Large oogonium showing surrounding stroma cells, a centrosome 

 with two rings of microsomes — the essential structure of an oocyte at the 

 beginning of growth. 



Fig. 5. Oogonium with surrounding stroma cells and centrosome previous 

 to division. 



Fig. 6. Spindle stage of the oogonium, first phase of the division period. 



Fig. 7. Two-cell stage of the oogonium previous to the second divteion. 



Fig. 8. Second division of the oogonium leading to the four-cell stage. 



Fig. 9. Four-cell stage of the oogonium after the second division. 



Fig. 10. A group of cells probably resulting from the third division of the 

 oogonia. 



Fig. 11. The first differentiation of the oocyte from follicle cells, showing 

 archoplasm and centrosome in the cytoplasm of the oocyte, and a central body 

 in the nucleus of the follicle. 



Fig. 12. Growing oocyte, showing traces of the centrosome and the cres- 

 cent-shaped archoplasmic body at one pole of the nucleus. 



Fig. 13. Section of growing oocyte at right angles to the egg axis through 

 the centrosome and sphere, showing its central position in the cytoplasm, 

 a very distinct sphere with its distinct central body, centrosome, in the 

 center of a lighter area. 



Fig. 14. Oocyte with pronounced polarity, showing its oval shape, and by 

 the position of the circle of microsomes with indistinct central granule, 

 its relation to the germinal vesicle, a relation which is maintained through- 

 out its succeeding history. 



Fig. 15. Oocyte showing the archoplasm forming a ring partly enclosing 

 the germinal vesicle, probably also the first beginning of a nucleolus in the 

 germinal vesicle. 



Fig. 16. Oocyte showing the relation of the centrosome and sphere to the 



