NUCLEAR DIVISION 



23 



ting lines gradually extend into the central part of the nucleus 

 and meet in its middle. They thus form a spindle-shaped group, 

 termed the nuclear spindle (Figs. 10, D, and 11), the two points 

 from which the indi\'idual streaks or fibres originate being called 

 the poles. It is debatable whether the spindle fibres represent 

 specialised strands of c^'toplasm, or are merely the expression 

 of physical forces operating within the cell. 



Since the chromosomes result from the breaking up of a 

 double thread, each is split longitudinally, but this feature is 

 often unrecognisable at this stage. The chromosomes now become 

 grouped in the equatorial region of the nuclear spindle, and 









Fig. II. — Mitosis (after Fraser and Snell). A, splitting of chromosomes. 

 B, equatorial grouping of same. Chromosomes are shaded. 



seem to become attached to some of its fibres (Fig. 11, A). In 

 the next stage these apparently shorten, and the two halves of 

 each chromosome become separated (Fig. 11, B), and begin to 

 move towards the opposite poles, which they ultimately reach 

 (Fig. 12, A). The diverging pairs of chromosomes often form 

 loops or V's, according to the mode of separation of the two . 

 halves, and during their passage to the poles each frequently 

 exhibits a longitudinal spHt comparable to that present in the 

 parent-chromosome. This spht probably forms the plane of 

 separation of the two halves at the next nuclear division. 



On reaching the poles the chromosomes give rise to two 

 daughter -nuclei (Fig. 12, B), which pass into the so-caUed resting- 



