THE CELL. 37 



nucleus is also absorbed. The vesicular character of the nucleus is 

 Lost during these changes in the arrangement of the chromoplasm, 

 which appears as a loose tangle or skein of one or more threads 

 of uniform diameter lying freely in the body of the cell. This 

 skein is called the spirem. The chromoplasm in this condensed con- 

 dition stains more deeply with nuclear dyes than in the resting con- 

 dition of the nucleus. The nucleoli in the meantime become faint 

 and seem to ultimately disappear. They play no part in the process 

 of cell-division, unless they participate in the formation of the 

 achromatic spindle. 



2. The Monaster Phase (Fig. 10). — The threads of the spirem 

 suffer a rearrangement, resulting in the formation of a sort of 

 wreath, situated midway between the poles, in the equatorial plane, 

 i. e., the plane perpendicular to and passing through the centre 

 of a line joining the two polar bodies. This wreath is called the 

 monaster, because of its star-like configuration when seen from 

 above. When viewed in profile it appears as a band of fibres lying 

 in the equator. It is at first made up of a single thread or only a 

 few threads, but subsequently breaks into a number of similar frag- 

 ments, called chromosomes. The exact number of these varies in 

 different species of animal, but is constant for each species and is 

 always divisible by two. In man it is thought to be sixteen. 



The chromosomes are all of nearly, if not quite, the same size, 

 and, in the same kind of cell, closely resemble each other in shape. 

 The most common form appears to be a V-shaped rod lying with 

 its angle directed toward the centre of the wreath or monaster. 



3. Metakinesis (Figs. 11, 12, 16). — In this phase of karyokinesis 

 the chromosomes split along their axes into two exactly equal parts 

 of similar shape, and these halves separate, each passing toward 

 one of the attraction-spheres. 



Meanwhile, the structure known as the achromatic spindle has 

 been formed. This is a system of fibres resembling those that have 

 already been described as radiating from the polar bodies, but of 

 even greater prominence. They are arranged to form a spindle 

 with its apices at the polar bodies and its equator coincident with 

 that of the cell and the plane of the monaster. 



It is along the lines of this spindle that the chromosomes travel 

 toward the centres of the attraction-spheres occupied by the polar 

 bodies. 



