66 MINUTE STRUCTURE OF CELLS LESS. 



nucleus is seen to contain a spindle-shaped body (sp] formed 

 of excessively delicate fibres which converge at each pole 

 to the corresponding directive sphere. The precise origin 

 of this nuclear spindle is uncertain : it may arise either 

 from the nuclear matrix or, more probably, from the 

 protoplasm of the cell : it is not affected by colouring 

 matters. 



At the same time each chromosome splits, sometimes 

 transversely, but usually along its whole length so as to 

 form two parallel rods or loops in close contact with one 

 another (B) : in this way the number of chromosomes is 

 doubled, each one being now represented by a pair. 



The divided chromosomes now pass to the equator of the 

 spindle (D) and assume the form either of V- shaped loops, 

 or of short rods, which arrange themselves in a radiating 

 manner so as to present a star-like figure when the cell is 

 viewed in the direction of the long axis of the spindle. 

 Everything is now ready for division to which all the fore- 

 going processes are preparatory. 



The two chromosomes of each pair now gradually pass 

 to opposite poles of the spindle (E, F), two distinct groups 

 being thus produced (G) and each chromosome of each 

 group being the twin of one in the other group. Probably 

 the fibres of the spindle are the active agents in this 

 process, the chromosomes being dragged in opposite 

 directions by their contraction. 



After reaching the poles of the spindle the chromosomes 

 of each group unite with one another to form a network (H) 

 around which a nuclear membrane finally makes its appear- 

 ance (i). In this way two nuclei are produced within a 

 single cell, the chromosomes of the daughter-nuclei, as well 

 as their attendant directive spheres, being formed by the 

 binary fission of those of the mother-nucleus. 



