LIFE HISTORY OF PINUS 3! 



arise in a plane perpendicular to the axis of the primary spindle. 

 Harper ('oo) makes the statement that in Larix, where no cell- 

 wall follows the first division of the pollen-mother-nucleus, the 

 spindle-fibers of the primary mitosis are utilized in the formation 

 of the spindle for the second division. I am unable to trace any 

 such connection in the pollen-mother-cells of Pinus, all traces of 

 the first karyokinetic figure having been lost to view before the 

 inception of the spindle for the second division. 



As the kinoplasmic fibers appear the chromatin-band forms a 

 double row of loops extending across the spindle-threads in the 

 plane of the equatorial plate. The longitudinal splitting is now 

 clearly apparent. The loops continue to shorten, and in this 

 position transverse fission occurs, segmentation almost always 

 taking place at the outer free ends of the loops (figs. 38 and 39, 

 plate IV). The sister-halves of each V- or U-shaped chromo- 

 some entirely separate, undergo readjustment, and finally come 

 to stand in a double row with their free ends in the line of the nu- 

 clear plate and their angles towards their respective poles (figs. 

 38-41). The spindle-fibers become attached to the chromosomes 

 at their point of bending, and the half chromosomes pass to the 

 poles (figs. 42-43). The dissociation of the sister-halves of each 

 segment is so complete before the beginning of the separation 

 at the equatorial plate that the figure during metakinesis is such 

 as to give the impression of whole chromosomes passing to the 

 poles, but a study of the prophases of the division shows clearly 

 that each represents the half of a double chromosome. In the 

 telophase of the division the chromosomes unite end to end to 

 form a spireme(fig. 44). The nuclear membrane appears, and 

 the chromatic band branches, giving rise to the reticulum of 

 the resting nucleus (figs. 44 and 45). 



The Problem of Reduction. Here as in all studies of spore- 

 formation at the present time the question of reduction demands 

 consideration. As already indicated, the reduction in the num- 

 ber of chromosomes takes place, as is the rule, during the so- 

 called resting stage of the spore-mother-cell, the one half num- 

 ber of chromosomes appearing in the prophase of the hetero- 

 typical division. But the inquiry concerning the presence or 

 absence of a qualitative reduction is not so easily answered. 



