30 VETERINARY PHYSIOLOGY 



length of the nucleus. The two centrosomes are now very 

 distinct, and from them a series of radiating lines 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. 4 (2)). 



Each loop now splits longitudinally into two, the divisions 

 lying side by side (fig. 4 (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 

 remaining 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. 4 (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 sinofle nucleus. Between these two nuclei a delicate line 

 appears, dividing the cell in two, and the division is com- 

 pleted (fig. 4 (4)). 



The network of the nucleus of actively dividing cells is 

 rich in nucleic acid, but in cells which have ceased to divide, 

 in which the nucleus has ceased to exercise its great repro- 

 ductive function, the amount of nucleic acid diminishes, and 

 may be actually less than the amount in the cell protoplasm. 



B. Amitotic Division. — In some cells the nucleus does not 

 appear to take an active part, the cell dividing without the 

 characteristic changes above discussed. 



