288 LILLIE. 



EXPLANATION OF PLATE XXV. 



Fig. 15. Earliest stage in the formation of the second maturation spindle. 

 The central spindle arises within the inner sphere. Each of the centrosomes is at 

 least quadruple. Radiations surround the entire sphere, and are not specially 

 related to the centrosomes of the new central spindle. The fibers of the aster of 

 the first maturation spindle form a reticulum with large, deeply staining microsomes. 



Fig. 16. Slightly later stage; the elongation of the central spindle has 

 stretched the sphere into an elliptical form. The boundary of the sphere forms 

 the periphery of the new central spindle, from all parts of which radiations proceed. 



Fig. 17. Further elongation of the central spindle ; the radiations of the pre- 

 ceding stages (Figs. 15 and 16) have become resolved into vesicular cytoplasm, 

 and new asters are forming in this from each of the compound centrosomes ; the 

 aster at the inner end of the spindle is much farther developed than that at the 

 outer end. Mantle-fibers are attached to the chromosomes, which are being 

 drawn towards the equator of the spindle. The centrosomes are larger and more 

 subdivided than in the preceding stages, but as they are somewhat flattened 

 radially this is not very evident in this figure. See, however, Fig. 24. 



Fig. 18. Later stage. The chromosomes are now nearly at the equator of the 

 spindle. The inner centrosome has nearly undergone the metamorphosis more 

 fully pictured in Figs. 24-27. The outer end of the spindle is in contact with 

 the surface, apparently as a result of the elongation of the spindle ; fibers pass 

 from the inner centrosome to the surface. 



Fig. 19. In this stage the outer centrosome still consists of a number of deeply 

 staining granules, each of which under ocular 18 seems to be compound. The 

 outer sphere is now established by the arrangement of the first row of microsomes. 



Fig. 20. The outer centrosome has now completed its metamorphosis into a 

 sphere (inner sphere) with central granule (centrosome). The chromosomes are 

 splitting longitudinally. For the sake of clearness only about one-third of the 

 chromosomes are drawn in. 



Figs. 21-23. Metaphase and anaphase of the second maturation spindle. 

 Note the peripheral distribution of the yolk in Fig. 23. 



Figs. 24-27. These are tangential sections through the outer centrosome of 

 the second maturation spindle in the stages of the spindles figured immediately 

 above them (Figs. 17-20). They are camera drawings with comp. oc. No. 8 and 

 hom. imm. apo. 2 mm. of Zeiss. 



Fig. 28. Accessory amphiaster. The inner end of the second maturation 

 spindle (in the anaphase), cut obliquely, is to the right. The sperm-nucleus in 

 this egg was found in another section near the side of the egg opposite to the 

 accessory amphiaster. 



Fig. 29. Figs. 29-31 illustrate the second mode of formation of the second 

 maturation spindle. In Fig. 29 the aster and centrosome of the first maturation 

 spindle have almost entirely disappeared. 



Fig. 30. The chromosomes have moved in towards the center of the egg, and 

 the central spindle is forming in a horizontal plane. Each end of the spindle is 

 occupied by a group of centrosome granules. 



Fig. 31. The spindle is completely formed and has swung into its definitive 

 radial position. At each end of the spindle is a large group of centrosome gran- 

 ules, which later undergo the metamorphosis shown in Figs. 24-27. There is no 

 such difference in the time of the metamorphosis of the centrosomes in this mode 

 of formation of the spindle as in the other mode. 



