46 TEXT-BOOK OF PHYSIOLOGY. 



cone, the apices of which correspond in position to the centro- 

 soma. This is known as the nuclear spindle. During the 

 prophase the nuclear membrane and the nucleoli disappear. 



2 The Metaphase. The two centrosomes are at opposite ends of 

 the long axis of the nucleus, each surrounded by an attraction 

 sphere, now called the polar radiation. The chromosomes 

 become yet shorter and thicker, and move toward the equator 

 of the nucleus, where they lie with their closed ends toward the 

 axis, presenting the appearance, when seen from the poles, of 

 a star, the so-called mother star, or monaster. While moving 

 toward the equator of the nucleus, and often earlier, each 

 chromosome undergoes longitudinal cleavage, the sister loops 

 remaining together for a time. Upon the completion of the 

 monaster, one loop of each pair passes to each pole of the nucleus, 

 guided, and perhaps drawn by the threads of the nuclear spindle. 

 The separation of the sister segments begins at their apices, 

 and as the open ends are drawn apart they remain connected 

 by delicate achromatin filaments drawn out from the chromo- 

 somes. This separation of the daughter chromosomes, and 

 their movement toward the daughter centrosomes, is called 

 metakinesis. As they approach their destination, we have the 

 appearance of two stars in the nucleus the daughter stars, or 

 diasters. 



3. Anaphase. The daughter stars undergo, in reverse order, much 

 the same changes that the mother star passed through. The 

 chromosomes become much convoluted, and perhaps united to 

 one another, the lateral twigs appear, and the chromatin resumes 

 the appearance of the resting nucleus. The nuclear spindle, 

 with most of the polar radiation, disappears, and the nucleoli 

 and the nuclear membrane reappear, thus forming two complete 

 daughter nuclei. Meanwhile the protoplasm becomes con- 

 stricted midway between the young nuclei. This constriction 

 gradually deepens until the original cell is divided, with the 

 formation of two complete cells. 



