No. 4-] BULLA SOLITARIA. 153 



these changes in the wall are taking place, the centrosome 

 becomes much enlarged and assumes an elliptical shape. Im- 

 mediately after these breaks appear, there is a small part of 

 the medullary area which takes a plasma stain. This area is 

 somewhat triangular in shape and occurs at the end of the 

 centrosome between the central corpuscle and the broken 

 periphery of the centrosome. New astral rays are formed 

 which extend to the central corpuscle. The old astral rays can 

 be seen disappearing at this stage in the cytoplasm. 



Fifth stage (Fig. 12). The central corpuscle of the second 

 polar spindle has enlarged and is still ^differentiated. The 

 pieces of the periphery of the old centrosome have become 



FIG. 8. FIG. 9. FIG. 10. FIG. n. FIG. 12. FIG. 13. 



FIGS. 8-13. The changes through which the central corpuscle and centrosome pass from the 

 prophase of the first maturation spindle to the metaphase of the second maturation spindle. 

 In each case the solid dark body is the central corpuscle. The granular area surrounding the 

 central corpuscle is the cortical zone. Figs, q, 10, and 13 show a complete centrosome, 

 having a central corpuscle and medullary zone. 



smaller. There is now a distinct cortical zone around the cen- 

 tral corpuscle, which has been derived from the medullary 

 zone of the centrosome of the first polar spindle. This stage 

 is identical with the first one described, except that the rim of 

 the old centrosome is still present and the central corpuscle is 

 only about one-half as large. 



The sixth stage (Fig. 13) shows the rim of the old centro- 

 some still present, but in smaller pieces which do not stain 

 as deeply as in the previous stage. The cortical zone has 

 enlarged and become fainter. The centrosome is composed of 

 a medullary zone and a central corpuscle. 



Summary. 



The central corpuscle of the first polar spindle becomes the 

 centrosome of the second polar spindle with a medullary zone 

 and a central corpuscle. The medullary zone of the centro- 



