12 DIVISION OF METAZOAN NUCLEUS 



of usually elongate structures called chromosomes, each of which splits 

 longitudinally into two daughter chromosomes, one of which passes into 

 each daughter nucleus. This division and separation of chromosomes is 

 associated with the formation of the achromatic figure which arises in con- 

 nection with a structure called the centrosome situated in the cytoplasm 

 outside the nucleus. The whole process can be regarded as talcing place 

 in a number of stages known as the prophase, metaphase, anaphase, and 

 telophase (Fig. 3). 



PROPHASE. — The centrosome, which is a spherical structure at the 

 centre of which is a deeply staining granule, the centriole, divides into two 

 parts which separate from one another. As they separate, the two 

 daughter centrosomes remain connected by fibres which are arranged as 

 a spindle, the spindle fibres, while similar fibres radiate into the cytoplasm 

 from the centrosomes (Fig. 3, B and C). Each centrosome with its 

 radiating fibres constitutes the aster. Within the nucleus the linin net- 

 work becomes arranged in what has been supposed to be a long coiled 

 thread in which the chromatin granules are embedded. This thread is 

 known as the spireme. Structures such as nucleoli and karyosomes may 

 break up and disappear, and any chromatin they contain becomes arranged 

 in granular form with the rest of the chromatin of the nucleus upon the 

 spireme. Finally, the nuclear membrane disappears, while the spireme 

 segments into a number of chromosomes (Fig. 3, C). It seems probable 

 that the conception of the spireme as a single long coiled thread is not 

 correct, and that from its first appearance it consists of a number of long, 

 intercoiled, separate segments which become distinct as they contract to 

 form the chromosomes, the name given to the separate parts into which the 

 spireme was supposed to divide. With disappearance of the nuclear 

 membrane the separate chromosomes, each of which can often be seen to 

 consist of two closely united parallel threads, arrange themselves in a 

 looped fashion round the equator of the spindle, and in the plane of this 

 equator in such a manner that the bend of each loop is directed towards 

 the centre and the two ends away from it (Fig. 3, D and E). The chromo- 

 somes, which have become shorter and thicker at the equator of the spindle, 

 form the equatorial plate. 



METAPHASE — The chromosomes, which are now arranged as the 

 equatorial plate, and each of which may consist of two closely apposed 

 parallel structures, divide longitudinally into daughter chromosomes, 

 which commence to move towards the pole of the spindle (Fig. 3, E). 



ANAPHASE.^The daughter chromosomes now separate completely 

 into two groups at the poles of the spindle. The natural interpretation 

 that they are drawn there by the action of the fibres of the spindle to which 



