56 MARGARET C. FERGUSON 



fibers pass into the nuclear cavity before the spireme is estab- 

 lished (fig. 100). Rarely, the nuclear membrane appears pushed 

 in irregularly along its entire lower margin, as indicated 

 in figs. 96 and 98 ; as a rule, however, there seems to be one 

 deep, sharp indentation along one side of which the nuclear wall 

 first gives way (figs. 99 and 100). With the initial steps in the 

 disappearance of the nuclear membrane the nucleolus is either 

 not apparent or, if still demonstrable, it stains but feebly. 

 When the membrane disappears along the entire lower portion 

 of the nucleus, the kinoplasmic threads press so closely against 

 it that it can not be definitely demonstrated whether it passes 

 into the cytoplasmic and the nuclear reticulum or becomes fib- 

 rous and contributes to the formation of the achromatic threads 

 (figs. loi and 102). The threads which have been packed so 

 closely against the wall of the nucleus now press into the 

 nuclear cavity and mingle with those which have entered from 

 below. And the dense, granular, cytoplasmic area from which 

 the threads diverge is gradually dissipated (fig. 103). 



With the disappearance of the wall along the lower part of 

 the nucleus, the achromatic nuclear network seems to undergo 

 a partial rearrangement. A portion of it is resolved into granu- 

 lar threads of more or less regularity which, in general, assume 

 a position parallel to the threads entering the nuclear cavity ; 

 some of them become attached directly to the ends of these 

 fibers, lose their granular appearance and doubtless contribute 

 to the growth of the elongating spindle-threads. 



As the spindle-fibers proceed in their development across the 

 nucleus the chromatic spireme collects in the region of the future 

 equatorial plate, and becomes more or less massed together. 

 At the same time it assumes an homogeneous aspect and gives 

 rise by segmentation to the chromosomes (figs. 101-104). Some 

 of the ingrowing spindle-threads may extend across the nucleus 

 to the nuclear membrane, which is still present on the upper 

 side of the nucleus, but by far the greater number unite some 

 distance below this membrane to form several poles, thus giving 

 rise to a diarch spindle which, like the karyokinetic figures 

 occurring during the development of the pollen-grain is multi- 

 polar at its upper extremity and unipolar, or nearly so, at its 



