LIFE HISTORY OF PINUS 57 



lower end. Gradually the poles of the upper portion draw 

 together, while the spindle is somewhat shortened Dy the lower 

 extremity of the threads being again resolved into granules. 

 Finally a true bipolar diarch spindle is formed with the V-shaped 

 chromosomes oriented at the equatorial plate. Each pole termi- 

 nates in a slight granular condensation. The upper pole has 

 never been observed to reach the nuclear membrane, but fre- 

 quently coarse granular threads extend from the pole to the 

 membrane of the nucleus, and apparently act as supports for 

 the upper pole (fig. 105, plate X). These are evidently formed 

 by a rearrangement of the linin reticulum. The nuclear mem- 

 brane persists along the upper side of the nucleus until the late 

 telophase of the division (figs. 101-103, plate IX, and 104-107, 

 plate X). 



As the chromosomes pass to the poles the central spindle 

 elongates, so that the daughter-nuclei are separated, as a rule, 

 by a greater distance than the length of the original spindle. 

 While this is characteristic of cell-division in general, it is occa- 

 sionally much exaggerated here, the daughter-nuclei being 

 apparently forced apart with considerable energy. The nucleus 

 which occupies the position nearest to the micropylar end of the 

 ovule often shows a deep indentation along its upper surface 

 as if a resistance had been met with in the peripheral layer of 

 cytoplasm (figs. 11 1, plate X, and 113, plate XI). Not infre- 

 quently the upper nucleus is found almost entirely separated 

 from the cytoplasm (fig. 112). This, however, maybe due to 

 mechanical rupture during sectioning and staining. No cell-wall 

 is ever formed, and in only one instance was a condensation of 

 the spindle-threads in the region of the cell-plate observed (fig. 

 no). The spindle may contract at or near its center during 

 its dissolution, thus presenting the appearance of an hour-glass, 

 or it may give rise to such a condition as that shown in fig. 

 113. These appearances, with various modifications, are not 

 uncommon in this mitosis in Pinus. Hertwig ('98) describes 

 and figures a very similar lengthening of the spindle-fibers in 

 ActinosfhcBrium. He also finds that the elongating spindle 

 finally bends along its median line so that the daughter-nuclei 

 come to lie near together in very much the same way as that 



Proc. Wash. Acad. Sci., July, 1904. 



