The maturation and fertilization of the egg of Cerebratulus. 473 



Plate 20. 



JFig. 15. First polar spindle. X '^^0. 



Fig. 16. A somewliat abnormal first polar spindle which has 

 become partially divided longitudinally. Cf. the outer pole of the 

 spindle in Fig. 19. X 750. 



Fig. 17. First polar spindle with the chromosomes dividing. Both 

 poles of the spindle possess a pair of centrosomes, one pair lying at 

 right angles to the other. X 1000. 



Fig. 18. First polar spindle. X '^^0. 



Fig. 19. First polar body forming. The two centrosomes in the 

 polar body are widely separated. Such a figure would result from the 

 further development of the spindle shown in Fig. 16. X 750. 



Fig. 20. First polar body nearly separated. But one of the two 

 centrosomes in the polar body is shown. The centrosomes at the inner 

 pole are separating preparatory to the formation of the second polar 

 spindle. X ^50. 



Fig. 21. The young asters of the second polar spindle. There 

 are two centrosomes in each aster. X 750. 



Fig. 22. A later stage in the formation of the second polar spindle. 

 A well developed spindle with a pair of centrosomes is seen in the 

 first polar body. X ^^0. 



Figs. 23 and 24. Two sections of the same egg, showing that the 

 centrosomes sometimes lie on that side of the sperm-nucleus which lies 

 opposite the egg-nucleus. The radiations of the inner pole of the 

 second polar spindle have nearly disappeared. 



Fig. 25. The sperm-nucleus with its pair of centrosomes and 

 enormously developed aster approaching the egg-nucleus. 



Fig. 26. The two germ-nuclei lie nearly in contact. To the right 

 and to the left are the two sperm-asters with exceedingly large, coarse 

 fibres. Owing to its more favorable position the aster on the left is 

 much larger than that on the right. The sperm-nucleus is the lower 

 of the two, and in the preparation is equal to the egg-nucleus in size. 

 Below the polar bodies may still be seen the darkly stained protoplasm 

 which was derived from the germinal vesicle. The centrosomes are 

 without centrospheres and lie naked in the cytoplasm. 



Fig. 27. A stage similar to the last, but the egg is cut very 

 obliquely. One of the germ-nuclei is shown with the pair of enorm- 

 ously developed sperm-asters, which, in this case, are of equal size. 

 The other germ-nucleus, which lies in the following sections, is in- 

 dicated only in outline. Reconstructed. 



Fig. 28. The germ-nuclei are fusing. The rays of the asters are 

 very irregular and broken, although they reach nearly to the periphery 

 of the egg. They are plainly degenerating. As in Fig. 26, the fibres 

 of the left aster are several times as long as those of the right. The 

 centrosomes, as well as the centrospheres, have entirely disappeared, 

 and are not to be found anywhere in the egg. 



31* 



