288 CELL-DIVISION 



The Formation of the Nuclear Spindle. — At the 

 time the chromosomes begin to be developed from the 

 chromatin net the cytoplasm around the nucleus presents 

 an unchanged appearance. As they proceed with their 

 development, fibers begin to appear at two sides of the 

 nucleus. They are formed from the cytoplasm itself 

 merely by a rearrangement of its structure. Each fiber 

 has one end directed at a point a short distance from the 

 nucleus, technically known as the pole of the spindle, and 

 the other toward the nucleus itself (Fig. 68.1). Some- 

 time during the formation of the nuclear fibers the 

 nuclear membrane disappears, thus leaving the chromo- 

 somes free in the cell. Some of the spindle fibers then 

 extend to them and become attached. The spindle fibers 

 converge to two poles on opposite sides of the nucleus, 

 and some fibers from each pole become attached to each 

 chromosome. The processes so far described belong to 

 the preparatory stage of division, technically known as 

 pro-phase. 



The Splitting of the Chromosomes. — After the 

 fibers are attached to the chromosomes the latter line up 

 and form a flat figure midway between the poles. Each 

 then spUts lengthwise to form two exactly similar halves 

 (Fig. 68.2). The split occurs in a plane at right angles 

 to the spindle so that the fibers from one pole are attached 

 to chromosome-halves on their own side. The respec- 

 tive halves next migrate toward the pole to which their 

 spindle fibers are directed. This stage of nuclear division 

 is called metaphase. 



The Anaphase (Fig. 68.3) covers the period during 

 which the chromosomes are moving toward the poles of 

 the spindle. It is easy to observe what occurs, but diffi- 

 cult to explain how it is brought about. Some observers 

 are of the opinion that the fibers undergo a contraction 

 and draw the chromosomes along with them. Others 

 think that the fibers are not solid at all but merely repre- 

 sent lines of force along which the fine protoplasmic 



