656 Balls .— The Mechanism of Nuclear Division. 
structures and connected with the modified portion of those structures 
which is called the centrosome. 
In the higher plants the centrosome is again absent, and the scheme of 
division (as interpreted below) is similar to that of the Infusoria. We may 
hope that the results to be obtained by study of the fate of the achromatic 
structures in higher plants will ultimately be translated into terms of the 
specialized centrosome of the lower plants and the animals. 
Cell-division in Cotton. 
While studying the cytology of the flower of Egyptian cotton 1 during 
the summer of 1905, I noted certain novel and curious structures in cells 
which were undergoing the reduction division. 
The staminal column turns yellow when the flower bud is still young, 
the anthers having previously been of a pale buff colour. Material taken 
about two days before this change of colour shows the microspore mother- 
cells in all stages of development, from synapsis to walled spores, in a single 
flower. 
The synaptic stage (Fig. 2, PI. LIV) is of the usual kind. The nucleus 
is comparatively small—about twenty microns in diameter—and contains 
a large plasmosome, which stains darkly with Heidenhain’s haemotoxylin. A 
lightly stained, densely tangled, coiled thread is connected to this nucleolus. 
The spireme thread appears to be continuous. The spireme thread then 
begins to open out, and becomes a looser tangle (Fig. 3). In it are em¬ 
bedded rows of darkly-staining granules, each one showing a distinct 
longitudinal bisection. 
The nucleolus next decreases in size, and the granules stain more 
darkly ; the shrinking of the nucleolus appears to be very rapid, for the 
stage is rare ; the darkening of the granules is localized to those portions of 
the thread which correspond to the future position of the chromosomes. 
Except in these portions the thread of the spireme is now split. 
Each of these clusters of darkened granules becomes a mass of chroma¬ 
tin, or bivalent chromosome. This chromosome is not, however, merely 
bisected as were the granules, but is also divided transversely to the axis 
of the spireme thread ; four perfectly distinct chromatic areas are thus 
formed, being the four univalent chromosomes which are to be distributed 
to the four microspores. These quarters, or daughter chromosomes, are 
roughly spherical, not elongated. By this time (Fig. 4) the nucleolus is no 
wider than the spireme thread, with which it is seen to be continuous on 
both sides, and it scarcely stains at all with haemotoxylin. All trace of it 
is lost shortly afterwards. 
1 Balls, W. Lawrence: The Sexuality of Cotton. Khecl. Agric. Soc. Yr. Bk., 1905. For 
systematic position of the variety studied (Afill) see Sir G. Watt’s Wild and Cultivated Cottons. 
