15 



This breaking up of the nucleus is accompanied by 

 the dissolution of its membrane, thereby permitting of 

 direct contact taking place between the nucleoplasm and 

 cytoplasm of the cell. Series of fine granulated 

 threads appear diverging from one polar centre towards 

 the median circumference of the cell, then converging at a 

 similar point in the opposite pole to form the well-known 

 achromatic nucleus spindle. In general several such 

 " polar " points are described as first appearing, giving a 

 multipolar spindle, which later becomes bipolar (Figs. 

 17-18). The origin of these spindle filaments is the 

 subject of various explanations. Strasburger thought they 

 were derived from the nucleolus, others that they were 

 threads of nuclear linin or possibly came from the cyto- 

 plasm. To these threads the chromosomes are attached, 

 and their movements appear to be regulated by them. 



Metaphase. — Whilst the chromosomes form the 

 nuclear-plate of the spindle they divide and the halves 

 separate ; of these one is drawn towards one pole, the other 

 to the opposite pole to form the chromosomes of the 

 daughter nuclei. This division really appears to be a 

 re separation of the end-to-end chromosome pairs that 

 united to form the " reduced " bivalent chromosomes of the 

 mother-cell. If so, the next — " homotype " — mitosis will 

 divide each of these single or univalent chromosomes and 

 distribute their respective halves to the grand-daughter 

 nuclei of the four pollen cells (Fig. 20). 



Anaphases. — These stages may be viewed as the 

 reverse of the later prophases. They have to do with the 

 movements of the separated chromosomes from the equator 



k towards opposite poles of the cell, there to be utilised in 

 the formation of a chromatin thread in each daughter 

 nucleus (Fig. 18). The spindle threads attached to the 

 chromosomes draw them apart and to the respective poles. 



