292 bulletin: museum of comparative zoology. 



centroplasm is well difFereutiated, whereupon it becomes smaller and 

 spherical. The intimate relation between the differentiation of the me- 

 dullary and cortical layers of the sphere, on the one hand, and that of the 

 centrosome and centriole, on the other, is noticeable in the indirect, as it 

 was in the direct process ; in every instance, with the one exception of 

 the outer pole of Figure 55, the medullary layer is not distinguishable 

 until the centroplasm can be seen. It is at this period that the secondary 

 rays of the asters become numerous. 



The changes in the indirect process have now been followed from the 

 first indication of the achromatic figure until the completion of the sec- 

 ond maturation spindle. The chromosomes of the second spindle have 

 divided (Fig. 55), but have not begun to migrate toward its poles, and 

 the achromatic figure has reached about the same stage of development 

 as the one last described (Plate 6, Fig. 36) in the shorter process. The 

 further maturation phenomena are the same in both processes, and hence 

 may be described together. In these changes there is nothing to indi- 

 cate whether the egg has passed through the direct or indirect process. 

 A comparison of Figures 36 and 37 (Plate 6) with Figures 54 and 55 

 (Plate 8) shows that a difference in size is the only real distinction. If 

 the long spindle of the indirect process did not become shortened, it 

 might serve as a distinguishing feature. 



(c) Further Changes in Maturation. 



The further maturation changes may be briefly described as follows : 

 In Ficfure 50 (Plate 8) the divided chromosomes have moved toward 

 their respective poles, and the interzonal filaments stretching between 

 them are easily seen. The centrosome at the inner pole is clear and 

 sharply outlined ; it has become somewhat compressed in the direction 

 of the axis of the spindle. The centrosome at the outer pole is small 

 and apparently much flattened in the same direction as the one at the 

 inner pole ; but it lies so close to the periphery of the egg that it is diffi- 

 cult to detei-mine its outline. There are a number of large, deeply stain- 

 ing granules scattered about in the cytoplasm surrounding the deep 

 centrosome which were not before distinguishable. 



The next older stage illustrated is shown in Figure 52. The chromo- 

 somes have moved apart still farther, and indeed have come so close to 

 their respective centrosomes as partly to cover them. The animal pole 

 of the egg has been raised up into a prominent conical elevation, near 

 the apex of which the outer centrosome and its cluster of chromosomes 

 are located. The latter are more closely crowded together than the cor- 



