Metaphase. The period of mitosis termed metaphase is, to speak precisely, the time when the two halves of the split chromo some separate from one another. However, this is a period of such short duration that for practical purposes nuclei are consid ered in metaphase when their chromosomes are lined up at the nuclear plate. The metaphase of mitosis is generally the most conspicuous of the nuclear activities not only on account of the position of the chromosomes (see Fig. 5), but because all kinoplasmic structures (the fimbrillae and centrosomes or centrospheres, if present) are shown to their best advantage.
The best evidence indicates that the chromosomes of plants may only divide longitudinally. This matter has considerable theoretical interest, which will be considered later and also in connection with the spore mother cell. The daughter chromosomes are drawn apart by the contraction of the fibrillae to which they are attached.
Chromosomes may take on various forms during metaphase, especially while they are being separated. Thus, if the chromosomes are dragged apart from the ends, their form is generally rod shaped ; but if the attachment of the fibrillae 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 introduced in the spore mother cell by certain premature divisions by which each daughter chromosome becomes a pair of granddaughter chromosomes instead of remaining a single structure. The peculiarities of the heterotypic and homotypic mitoses are due
FIG. 5. Metaphases of Mitosis, a, Saprolegnia ; intranuclear spindle in oogonium, nucleolus outside of spindle. 6, Erysiphe ; mitosis in ascus, asters with rather small centrospheres. C, Corallina ; first mitosis in tetraspore mother-cell, very large and well differentiated centrospheres. D, Zamia ; blunt poled intranuclear spindle in central cell of pollen grain ; blepharoplasts, their outer membrane about to break up. E, Pellia ; first mitosis in spore mother-cell; broad spindle with rounded poles, the very numerous spin dle fibers ending in granular kinoplasm. /, Agave; first and second mitoses in pollen mother-cells ; (i), multipoiar spindle just previous to metaphase ; the several independent cones of fibrills gather more closely together to complete the spindle. (2) metaphase of second mitoses ; completed spindles showing however the several independent cones of fibrills. (After Harper, Webber and Osterhout.)
