OF THE CELL THEORY 175 



by the splitting of a single primary thread are 



spoken of as " sister-threads." As they are of 



exactly equal size, and as each of the primary 



threads becomes divided into a pair in this way, 



the upshot of the process is that the whole of the 



chromatin becomes divided into two precisely equal 



portions, a process the full significance of which will 



become apparent immediately. About or shortly 



after the time of the splitting of the primary threads 



into sister-threads, a structure known as the 



nuclear spindle appears. This is a fusiform figure, 



bounded by a number of exceedingly fine and 



feebly staining threads. It is said to be formed 



not from the nucleus, but from the cell protoplasm 



immediately surrounding the nucleus. At the time 



of its first appearance it has no clear relation to the 



looped threads of the nucleus, but very soon the 



two structures take up definite positions with 



regard to each other, the looped threads being 



grouped in a ring round the equator of the spindle, 



with the loops directed inwards towards the 



centre and the free ends outwards. At the same 



time the spindle itself becomes more clearly 



marked; at its poles are two rounded bodies, the 



" pole-bodies " of van Beneden, from which fine 



threads radiate outwards in all directions through 



the protoplasm of the cell-body. The nuclear 



membrane disappears about this time, though what 



exactly happens to it is unknown. It was formerly 



supposed to give rise, at any rate in part, to the 



nuclear spindle, but it is now generally agreed that 



the spindle is extra- nuclear and a product of the 



