CELL-DIVISION AND NUCLEAR DIVISION 57 



cell of a Higher Plant, must be considered in some detail. The process 

 of cell-division comprises two separate series of change, which must be 

 carefully distinguished, namely, first, the partition of the protoplast 

 which generally involves the formation of a cell- wall between the two 

 daughter-cells and, secondly, the division of the nucleus. These two 

 processes are correlated with one another in a peculiar and characteristic 

 manner, a circumstance which complicates the problem at issue. Never- 

 theless, thanks to the laborious researches of Flemming, Strasburger 

 and numerous other investigators, the leading features of the process of 

 cell -division are comparatively well understood. 



When a cell is about to divide, the nucleus may be seen to undergo 

 certain preparatory changes. The threads of the nuclear reticulum 

 become thicker and shorter (Fig. 7 a), while at the same time the 

 chromatin granules enlarge and assume the form of discs which are 

 placed athwart the threads. The tangled thread or skein next resolves 

 itself into a definite number of segments or chromosomes (Fig. 7 c), 

 which usually take the form of U - or L-shaped rods. The chromosomes 

 then arrange themselves in a single plane, the equatorial plane, collec- 

 tively forming the so-called equatorial plate : the bend of each U or L 

 always faces the equatorial plane. At this point it becomes obvious 

 that each chromosome is divided longitudinally into two equal halves ; 

 this fission, however, is initiated at an earlier stage. 



While the nuclear network is contracting and breaking up into 

 chromosomes, other changes are also going on. The nuclear membrane 

 becomes invested by delicate plasmatic strands which appear first at 

 the two poles of the " mitotic figure " in the shape of the so-called polar 

 caps ; similar protoplasmic fibrillae subsequently become differentiated 

 in the interior of the nucleus, where they take the form of two bundles 

 tapering towards the poles (Fig. 7 b, c). Later the nucleolus and the 

 nuclear membrane disappear, while the fibrils of the polar caps become 

 prolonged inwards and attach themselves to the backs of the chromo- 

 somes ; other fibrils derived from the two caps meet end to end and 

 fuse to form threads running continuously from pole to pole. All 

 these protoplasmic threads are termed spindle fibres ; collectively they 

 constitute the nuclear spindle. As stated above, the chromosomes, after 

 forming the equatorial plate, undergo longitudinal fission ; the two 

 halves of each chromosome then travel along the spindle-fibres to 

 opposite poles (Fig. 7 d), where they proceed to form the daughter- 

 nuclei. For this purpose the chromosomes withdraw their freely 

 extended ends and give rise by involution, branching and anastomosis 

 to a typical nuclear reticulum. In the meantime a new nuclear mem- 

 brane has developed, and so the reorganisation of the daughter-nuclei 

 is complete. The nucleoli either disappear altogether before the 



