68 THE CELL. 



somes are now free in the protoplasm ; gradually the crown of each 

 chromosome approaches the center of the space occupied by the 

 nucleus, and the chromosomes form a characteristic, radially 

 arranged stellate figure, known as the monaster, in the equatorial 

 plane of the cell. During the progress of the changes affecting the 

 chromatin of the nucleus and resulting in the formation of the 

 chromosomes, important phenomena are observed, connected partly 

 with the achromatic substance of the nucleus, more especially with 

 the centrosome, centrosphere, and cytoplasm of the cell. These 

 phenomena result in the formation of a complicated structure known 

 as the acliromatic spindle or amphiaster. Its development is as fol- 

 lows : The centrosome and centrosphere, as has been stated, usu- 

 ally lie in the protoplasm to one side of the nucleus. If, at the be- 

 ginning of the division, the centrosome be single, it divides, and the 

 two centrosomes begin to separate, causing a division of the centro- 

 sphere. Between the centrosomes are usually seen finely drawn-out 

 connecting threads. The centrosomes, each of which is surrounded 

 by a centrosphere, now move apart, and a structure known as the 

 central spindle, and consisting of fine threads arranged in the form 

 of a spindle, develops between them. At each end of the central 

 spindle is found a centrosome surrounded by a centrosphere from 

 which radiate into the cytoplasm fine fibers known as polar rays. 

 During the formation of the achromatic spindle the nuclear mem- 

 brane disappears and the chromosomes develop, as above described. 

 Some fibers, which seem to have their origin from the centrosphere, 

 grow into the spirem formed of chromosomes, which they appear to 

 pull into the equatorial plane of the cell, which is also the equator 

 of the central spindle. Thus, the nuclear figure above described 

 as the monaster is formed. In other cases the centrosomes 

 and centrospheres continue moving apart until opposite each other 

 and separated by the nucleus (Figs. 23, 24). As the nuclear 

 membrane disappears and the spirems and chromosomes are form- 

 ing, the central spindle develops, its fibers running from centro- 

 sphere to centrosphere. The polar rays also develop in the cyto- 

 plasm at the same time. As the central spindle develops, the 

 chromosomes arrange themselves or are arranged about its equator 

 monaster. 



(//) Metaphases. Usually, during the formation of the monaster, 

 or immediately after its formation (sometimes in the spirem stage or 

 even earlier), the most important process of cell -division takes 

 place. Each chromosome divides longitudinally into two daughter 

 chromosomes. The loops first divide at the crown, the cleft extend- 

 ing up either limb until the free ends are reached. The smallest 

 particle of chromatin divides, retaining the exact relative' position in 

 the twin chromosomes that it possessed in the mother chromo- 

 some. The daughter chromosomes now wander over the central 

 spindle, their crowns presenting, in opposite directions toward the 

 poles of the cell. This process is known as metakinesis. Two stel- 



