Page:Popular Science Monthly Volume 26.djvu/833

Rh can understand their disappearance, for the fibrils lose their net-like reticulation, and become an irregular convolution, with no free ends. Around this fibrous nuclear mass appears a clear space, which separates it from the outer cell-substance. As thus arranged it forms what has been called the aster.

Soon the fibrous convolutions assume a wreath-like arrangement, with their bends irregularly directed toward a central space. Eventually the wreath loses its continuity, and breaks into a series of short, separate fibers, which form V-shaped loops. The bends of these loops are directed toward the center space, their openings outwardly. This arrangement forms the mother-star. Next there is shown a doubtful appearance, as if the fibers had split into two, or had become tubular. The loops are also compressed toward the equatorial plane of the nucleus, and lose their extension toward its polar region. After some further dubious movements, a rearrangement of the loops is found to have taken place, their bends being now turned outward, their openings inward toward the equatorial plane. They have also separated in this plane, so as to form two distinct masses, one on each side of the equator. If we consider the cell as a globe, and the equatorial plane as a circular disk dividing this globe into two hemispheres, then on each side of this disk lies a smaller circle of fibrous loops, which present something of the aspect of a circular basket, or of a partly opened daisy. The openings of these basket-like figures are turned toward each other, with the equatorial plane separating them. The converging looped ends of the fibers are turned outward.

This stage in the process of division of the nucleus is followed by a recession of the basket-figures. They retreat in the axial line of the cell until they reach the polar regions of the nucleus. Here a rearrangement of the fibrous loops takes place, their bends again become directed toward a central space, and two new stars, similar to the mother-star, are formed. The division of the chromatin, or fibrous substance of the nucleus, has become complete, and the whole new arrangement is known as the dyaster.

As the basket-like figures recede, there often appear in the interval between them delicate striæ, which cross the equator from pole to pole. This condition, which is most declared in vegetable cells and in segmenting ova, is known as the nuclear spindle. The lines of the striæ seem to be composed of achromatin. Other faint lines often radiate from the poles toward the surface of the cell, forming sun-like figures at the extremities of the nuclear spindle. Complete division is preceded by the appearance of a row of dots across the equatorial plane, which seem to be thickenings in the centers of the lines of the spindle. These thickenings are probably composed of chromatin, and form what is called the equatorial plate. They soon divide, the spindle separating in its center, while the thickenings appear like minute disks at the extremities of the nuclear striæ. Thus a double