20 HUMAN PHYSIOLOGY 



actual fibres or they may be lines of movement in the proto- 

 plasm. The spindle passes into the centre of the nucleus, and 

 seems to direct the changes in the reticulum. The nuclear 

 membrane disappears, and the nucleus is thus not so sharply 

 marked off from the cell protoplasm. The nucleoli and 

 nodal points also disappear, and with them all the finer 

 fibrils of the network, leaving only the stouter fibres, which 

 are now arranged either in a skein or as loops with their 

 closed extremity to one pole of the nucleus and their open 

 extremity to the other. The nucleus no longer seems to 

 contain a network, but appears to be filled with a con- 

 voluted mass of coarse fibres, and hence this stage of nuclear 

 division is called the convoluted stage. 



The spindle continues to grow until it occupies the whole 

 length of the nucleus. The two centrosomes are now very 

 distinct, and from them a series of radiating striae extends 

 out into the protoplasm of the cell. 



The nuclear loops of fibres break up into short, thick 

 pieces; and these become arranged around the equator of 

 the spindle in a radiating manner, so that when the nucleus 

 is viewed from one end it has the appearance of a rosette or 

 a conventional star. This stage of the process is hence often 

 called the single star or monaster stage (Fig. 3, 2). 



Each loop now splits longitudinally into two, the divisions 

 lying side by side (Fig. 3, 2). 



The next change consists in the separation from one 

 another of the two halves of the split loops one half of each 

 passing up towards the one polar body, the other half passing 

 towards the other. It is the looped parts which first separate 

 and which lead the way the open ends of the loops remain- 

 ing in contact for a longer period, but, finally, also separating. 

 In this way, around each polar body, a series of looped fibres 

 gets arranged in a radiating manner, so that the nucleus 

 now contains two rosettes or stars, and this stage of division 

 is hence called the dy aster stage (Fig. 3, 3). 



The single nucleus is now practically double. Gradually 

 in each half finer fibres develop and produce the reticular 

 appearance. Nuclear nodes, nucleoli, and the nuclear mem- 

 brane appear, and thus two resting nuclei are formed from a 

 single nucleus. Between these two nuclei a delicate line 



