THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 



21 



The first observed step in cell division is binary division of the 

 attraction-particle: its halves evolve a set of very fine achromatin 

 filaments uniting them, so that each half is one of the poles of a 

 spindle-shaped collection of fibers, the nuclear spindle. Mean- 

 while the nucleolus and nuclear membrane disappear, being prob- 

 ably taken up into the rest of the chromoplasm, which now, in- 

 stead of its original reticular arrangement, takes the form of a 

 single long chromatic filament coiled in the nucleoplasm. At one 



FIG. 9. Diagrams of a nucleus in an early stage of karyokinesis. A showing 

 the polar, B the antipolar region; a, nuclear or achromatin spindle; 6, part of 

 general cell-protoplasm around the nucleus; c, looped chromatic filament; d, 

 nucleoplasm. 



portion of the nucleus (pole) the loops of the chromatic filament 

 leave a spiace free from them (Fig. 9, ^L), and in the neighborhood 

 of this space the nuclear spindle is first seen within the nucleus. 

 At the opposite side of the nucleus or antipole (Fig. 9, B) the loops 

 of the chromatic filament leave no clear space, but cross irregu- 

 larly. In the next stage the loops at the antipolar end break 

 through, and in this way the filament is divided into a number of 

 irregular elongated Vs, each with its closed angle near the pole 

 and its open end near the antipole. The spindle meanwhile passes 

 to the center of the nucleus and takes a position in which its long 

 axis coincides with that joining pole and antipole, and then the 

 Vs of chromoplasm become shorter and their limbs thicker, and 

 they also shift position so as to group themselves radially around 

 the equator of the spindle (A, Fig. 10) with their angles directed 

 centrally. Each V then divides along its whole length, and one- 

 half passes towards the pole, the other towards the antipole. The 

 whole nucleus elongates in the direction of the long axis of the 



