LIFE HISTORY OF PINUS II 9 



shaped opening. While this opening is still inconspicuous, the 

 two halves of a given chromosome become distinct throughout 

 the entire length of the segment. Such a condition was several 

 times observed in the division of the " segmentation-nucleus," 

 but was not sketched because of lack of space. A similar stage 

 in the division of one of the four nuclei of the proembryo is 

 shown in fig. 253, 3, plate XXIII. 



In general the chromosomes of the nuclear plate are in the 

 form of U's and V's ; in rare instances they are long and some- 

 what coiled, and the spindle-fibers are not attached to their cen- 

 ters (figs. 234-238). They pass to the poles as narrow U's 

 (fig. 239). Sometimes the arms of the U are pressed so closely 

 together that the chromosomes look like longitudinally split 

 rods. In a late anaphase of the division the chromatic ele- 

 ments present a crinkled appearance, and the poles of the spin- 

 dle terminate in granular areas from which threads extend into 

 the surrounding cytoplasm. These fibers may be quite incon- 

 spicuous or they may be very prominent, frequently forming 

 fantastic figures (figs. 240 and 241). 



A portion of the achromatic constituents of the sexual nuclei 

 may persist in the region of the mitotic figure until the forma- 

 tion of the daughter-nuclei, but, as a rule, all traces of the 

 original nuclei have disappeared at this time. Blackman finds 

 no suggestion of a cell-wall in connection with the first division 

 which takes place within the oosphere. But here, again, I have 

 found great variation. The spindle either becomes constricted 

 at the center with little or no sign of thickening along its median 

 line, or it may be very broad, in which case prominent thicken- 

 ings occur, only to disappear at a later stage, in the line of the 

 cell-plate (figs. 239 and 242). As the half chromosomes unite to 

 form the daughter-nuclei the poles of the spindle often become 

 very slender and seem to press against the forming nuclei, ren- 

 dering them concave along their inner surfaces ; and delicate 

 fibers now extend from all sides of the division-figure into the 

 cytoplasm (fig. 242). As already indicated, there is no evi- 

 dence that any portion of this spindle is derived from the cyto- 

 plasm, and it is probable that a large part, if not all, of its 

 fibers are formed by a rearrangement of a portion of the achro- 



