446 



THE AMERICAN NATURALIST. [Vol. XXXVIII. 



especially while they are being separated. Thus, if the chromo- 

 somes are dragged apart from the ends, their form is generally 

 rod shaped ; but if the attachment of the fibrillse is near the 

 middle of the chromosome, the structures are pulled apart as 

 loops or V's, and the pair of chromosomes just previous to their 

 separation may be ring shaped. A further complication is intro- 

 duced in the spore mother cell by certain premature divisions by 

 which each daughter chromosome becomes a pair of granddaugh- 

 ter chromosomes instead of remaining a single structure. The 

 pecuharities of the heterotypic and homotypic mitoses are due 



Fig. 6. — Anaphase of Mitosis, , spore mother-cell of LUium mariagon. u, immediately 

 after metaphase of first mitosis; each daughter chromosome consists of two grand 

 daughter segments, adhering at the ends, making the familiar V-shaped figures charac- 

 teristic of the first mitosis (heterotypic) in the spore mother-cell of higher plants. It, late 

 anaphase of the first mitosis ; the V shaped chromosomes, each composed of two grand 

 daughter segments adhering at the ends, are very close to the poles of the spindle ; the 

 central fibers of the spindle are conspicuous at this stage, c, second mitosis ; the grand 

 daughter chromosomes, that composed the Vs of the first mitosis, have separated at the 

 nuclear plate of the second mitosis and are being drawn by their ends to the poles of the 

 spindle where they will organize the nuclei of the pollen grain; this mitosis is called 

 homotypic to distinguish it from the usual (typical mitoses in which there are no premature 

 divisions of the chromosomes. All figures after Mottier. 



to this phenomenon. (See account of spore mother cell in Sec- 

 tion III. 



y4«a/,^aj^.— Anaphase begins with the separation of the 

 daughter chromosomes at the nuclear plate (Fig. 6 ) and ends 

 with the gathering of these structures at the poles of the 

 spindle preparatory to the organization of the daughter nuclei. 

 As the chromosomes move towards the poles the fibers of the 

 central spindle stand out sharply (see Fig. 6 b). If a cell wall 

 is to be formed between the daughter nuclei one may expect to 

 find these fibers thickening in the equatorial region of the 



