THE CELL. 21 



threads are somewhat thicker than before, and more separated than 

 during the resting stage. With the formation of the spirem, or 

 wreath, the nucleoli and membrane, if any, disappear. In some cases 

 the nucleoli are dissolved and cast into the hyaloplasm, where they 

 degenerate and have no further function. 



The thread of the spirem becomes divided transversely into 

 nearly equal parts, or bodies, known as chromosomes, which, in most 

 cases, are in the form of rods, straight or curved. The ground- 

 substance of the nucleus now becomes a part of the surrounding hyalo- 

 plasm. The chromosomes at first are placed rather irregularly, but 

 they soon begin to arrange themselves into a more definite form, that 

 of a rosette. The curved chromosomes now become more angular 

 and V-shaped, the angle pointing toward the center of the nuclear 

 space while the free ends are directed toward the circumference, this 

 figure being called the aster, or garland. While in the form of the 

 aster each chromosome splits longitudinally into halves, so that we 

 have just again as many, though thinner, chromosomes. 



Before the membrane has been dissolved, there appear in the 

 protoplasm, but very near the nuclear membrane, two small granules 

 lying side by side. These are the centrosomes. They are of a sub- 

 stance that stains with difficulty. Gradually they begin to separate 

 from one another, moving in a semicircle, until they are diametrically 

 opposite one another, or at the nuclear poles. While they have been 

 in motion, the nuclear membrane has been dissolving, so that, by the 

 time they are again at rest, the membrane has disappeared. The 

 achromatic nuclear spindle develops between the centrosomes. When 

 they begin to separate, the spindle is small, scarcely discernible, and 

 like a band in form. As the centrosomes separate more widely, the 

 fibers become more plainly visible and assume the form of a spindle 

 broad in the middle and converging at either end, toward and end- 

 ing in the centrosomes. The protoplasm now arranges itself around 

 the centrosomes in the form of rays of a star, as though the filaments 

 of protoplasm were attracted by the centrosomes in the manner of 

 iron filings by a magnet. At first these fibers are small, but increase 

 in length and numbers as the division of the cell progresses, until they 

 run throughout the entire protoplasmic mass. 



The V-shaped filaments, called chromosomes, are now collected 



in the plane of the equator, called the equatorial plate. While the 



chromosomes have been arranging themselves in the plane of this 



plate, they have been growing somewhat shorter and thicker, their 



.angles pointing to the axis of the spindle and their ends to the cir- 



