58 MARGARET C. FERGUSON 



shown in fig. 113. I am unable to trace definitely the origin of 

 this figure, but it is not improbable that it is caused by a con- 

 traction of the cytoplasm resulting from the cessation of the 

 force which effected the separation of the daughter-nuclei ; or 

 it may be produced by the resistance which the peripheral layer 

 of cytoplasm, along the outer surface of the upper nucleus, 

 offers to the growing fibers, thereby forcing them back upon 

 themselves as shown in the figure. When all traces of the 

 spindle have disappeared, the two sperm-nuclei are surrounded 

 by a common mass of cytoplasm, and there is never throughout 

 the later history of this cell the least suggestion of a dividing 

 wall. 



The mitosis just described seems to be unique as regards the 

 origin and development of the achromatic spindle. Hertwig's 

 ('98) fig. 3, plate V, illustrating an early stage in the division 

 to form the first polar body in Actmospkarium, bears a striking 

 resemblance to the prophase of this mitosis as illustrated in fig. 

 95, plate IX, of this paper ; but the origin of the figure shown by 

 Hertwig, and the later history of the division are very dissimilar 

 to that of the karyokinesis under consideration. The most 

 exaggerated instances of asymmetry in spindle-formation which 

 I have found recorded as occuring in plants is that described 

 and figured by Nemec ('99 2 ) in Solanum tuberosum, and more 

 recently by Murrill ('oo) in the division of the central cell in 

 Tsuga. In both these instances the nucleus lies at one side of 

 the cell, and the spindle-fibers are very much more prominent 

 on the free side of the nucleus than on the side adjacent to the 

 cell-wall. In another paper Nemec ('99 3 ) shows by experimenta- 

 tion that the form of the figure which gives rise to the extra- 

 nuclear spindle depends upon external forces or conditions. In 

 obedience to the law established by Haberlandt ('87) we should 

 expect to find the generative nucleus in that part of its cell which 

 is nearest the growing point of the pollen-tube, rather than at 

 the end more remote from it, and it may be that its passage from 

 the lower to the upper side of the cell is due to the fact that the 

 forces, instrumental in effecting the division, first become active 

 at a point below the nucleus, and exert a repelling action on it. 

 But I have at present no adequate explanation or theory to offer 



