INDIRECT DIVISION 



17 



with one another; and Rosen states that each chromatin granule is com- 

 pletely imbedded in a broad strand of linin. Davis similarly interprets 

 the spireme shown in Fig. 20, F. Whatever the actual structure may be, 

 the chromatin granules in the spireme thread early divide in two, so that 

 the thread appears double. When the thread shortens and condenses to 

 form the chromosomes, the rows of granules may coalesce so as to pro- 

 duce a rod already divided lengthwise, although its halves are in close 

 apposition. Occasionally the ends of the chromosomes are seen to be 

 slightly separated. 



Metaphase, In the metaphase (Fig. 12, E), the two longitudinal 

 halves of each chromosome are being drawn apart toward the opposite 

 poles of the cell. If the chromosome is V-shaped, the separation of the 

 two halves begins at the apex of the V. 



At this stage an achromatic figure, known as the spindle, is evident 

 in plant cells, but it is more sharply defined in those of animals. As seen 

 in the diagram (Fig. 13), it consists of fibrils which pass from the equatoria 



Polar radiation. Nuclear spindle. 



FIG, 13. EARLY METAPHASE. 



FIG. 14. LATE METAPHASK. 



plate toward either pole, where, in animal cells, there is a well-defined 

 granule, the centrosome. Around each centrosome there are radiating 

 protoplasmic fibrils, forming the polar radiation (Figs. 13 and 14). The 

 polar radiation is also called an aster, and the two asters connected by 

 the spindle are known as the amphiaster. Some of the spindle fibers 

 are attached to the chromosomes and appear to pull their halves apart; 

 others pass from pole to pole without connecting with the chromosomes. 

 In animal cells the spindle arises as the two centrosomes, lying beside 

 the nucleus, move apart (Fig. 20, A). As they pass to the opposite poles 

 of the nucleus, the spindle forms between them, either from the nuclear 

 reticulum, or the cytoplasmic reticulum, or in part from both. These 

 conditions appear to vary in different animals. 



In the cells of root tips, a condensation of protoplasm forms a cap at 

 the poles of the nucleus at the time when the nuclear membrane and 

 nucleoli are disappearing. From the "polar cap," spindle fibers develop 



