174 



ON THE INTERNAL EORM AND 



[CH. 



there each becomes surrounded by a system of radiating Unes, the 

 asters; immediately around the centrosome is a clear space, the 

 centrosphere (Figs. 43-45). Between the two centrosomes with 

 their asters stretches a bundle of achromatic fibres, the spindle. 



3. The surface-film bounding the nucleus has broken down, 

 the definite nuclear boundaries are lost, and the spindle now 

 stretches through the nuclear material, in which he the chromo- 

 somes (Figs. 45, 46). These chromosomes now arrange them- 

 selves midway between the poles of the spindle, where they form 

 what is called the equatorial plate (Fig. 47). 



4. Each chromosome splits longitudinally into two : usually 



equatorial plate 



spindle fibres 



Fig. 46. 



Fig. 47. 



at this stage, — but it is to be noticed that the splitting may have 

 taken place so early as the spireme stage (Fig. 48). 



5. The halves of the split chromosomes now separate from 

 one another, and travel in opposite directions towards the two 

 poles (Fig. 49). As they move, it becomes apparent that the spindle 

 consists of a median bundle of "fibres," the central spindle, running 

 from pole to pole, and a more superficial sheath of "mantle- 

 fibres," to which the chromosomes seem to be attached, and by 

 which they seem to be drawn towards the asters. 



6. The daughter chromosomes, arranged now in two groups, 

 become closely crowded in a mass near the centre of each aster 



