224 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Figure 1). The " polar depression " described by Kostanecki und 

 Wierzejski (*96) for Physa is also not a rare phenomenon in Limnaea. 

 Couklin ('94) suggests that the flattening of the animal pole preceding 

 the formation of the first polar cell is caused by the contraction of the 

 spindle fibres. Perhaps, in a similar way the " polar depression " is 

 produced by a more active contraction of the spindle fibres which lie in 

 the prolongation of the axis of the spindle. But even before the outer 

 centrosphere has reached the cell membrane (Plate 1, Figure 1), the 

 polar depression has disappeared, and a marked flattening of the centro- 

 sphere accompanies that of the animal pole of the egg. On either side 

 of the outer centrosphere, as seen in optical longitudinal section of the 

 spindle (Figure 1), there is a projecting ''wing" of deeply staining 

 substance. Careful examination shows these wings to be composed of 

 closely crowded astral rays, which have stained near their proximal ends. 

 Since the "wings" are to be seen in all longitudinal sections of this 

 spindle, it follows that the appearance is due to the presence of a 

 continuous disk of staining substance. 



Studying the difiierent figures of the first maturation spindle of 

 Limnsea with special regard to the deeply staining portion at the poles, 

 one must, I think, conclude that both centrospheres sliown in Figure 1 

 have taken the stain throughout their whole extent. The roughness of 

 the outline of the stained centrospheres, as compared with those of 

 smaller size, is due to the increasing distance between the astral rays, 

 as one passes outward from the centre. This condition suggests the 

 idea that even parts beyond the limit of the centrosphere may have 

 been stained ; for if only the centrosphere were stained, the outline 

 should be more regular. Those investigators who have discovered 

 enormous centrosomes should examine Plate 4, Figure 23. The dense 

 mass shown there at each pole of the spindle looks more like a precipi- 

 tate lodged about a central point than like an organ of the cell. In 

 Figure 24 the outlines of the centrospheres may be seen. The great 

 irregularity in the form of the stained portions proves beyond question, 

 it seems to me, that the dense masses at the poles of the spindles repre- 

 sented in Plate 1, Figures 1 and 3, and in Plate 4, Figure 23, are 

 simply portions of the cell protoplasm which have not been decolorized. 



Many investigators, notably Mark, Garnault, and Kostanecki und 

 Wierzejski, have given in detail for pulmonates the process of the actual 

 formation of the polar cells. It is my purpose in this division of the 

 pnper to note only the modifications of the centrosome and centrosphere. 

 In the part devoted to the discussion of the nucleus, I shall make special 



