14 DIVISION OF METAZOAX NUCLEUS 



original nucleus (Fig. 3, G and H). The centrosome remains outside the 

 nucleus, the fibres of the aster becoming no longer visible. 



The centrosome appears to be the ruling factor in the process, and 

 the appearance of the aster and spindle fibres can be interpreted as visible 

 indications of some force which is being exerted. It must be remembered, 

 however, that in the mitotic division of the nuclei of the higher plants, 

 as also that of many Protozoa, though all the stages of mitosis seen in the 

 animal cell occur, definite visible centrosomes are not present. The 

 fibres of the aster and spindle radiate from an apparently structureless 

 area, which may be regarded as a potential centrosome. An important 

 fact to be noted is that for any particular species the number of chromo- 

 somes present in the nucleus of any cell of the body is constant. In the 

 much studied cells of Ascaris ntegalocephala, of which there are two 

 varieties, the number of chromosomes is two or four respectively. In 

 man it is twenty-two, while in other animals it may be much higher 

 than this. Each species of animal has thus a definite chromosome 

 number. 



The chromosomes which are formed in any nucleus are not necessarily 

 all alike in size or form. It is often found that they can be grouped in 

 pairs, the members of each pair resembling one another more closely than 

 those of other pairs. The members of each pair are known as homologous 

 chromosotnes. During the progress of mitotic division the chromosomes 

 are at first elongate structures, but there is a tendency for them to shorten, 

 so that at the stage when the equatorial plate is formed they may be 

 roughly spherical. Though these alterations in size take place, all the 

 chromosomes are similarly aft'ected. Their relative size and shape remain 

 the same, so that the homologous pairs can still be recognized. 



During the telophase, when the chromosomes of the daughter nuclei 

 are becoming transformed to reproduce the structure of the resting nucleus, 

 it can sometimes be seen that the chromatin and achromatic material 

 of each chromosome is occupied in reconstructing a j^articular portion 

 of the nucleus. When chromosomes are re-formed at the next nuclear 

 division, the material in each portion concentrates again into a chromo- 

 some. In these cases it appears as if there is a permanent separation of 

 the constituents of each chromosome, even when the nucleus is in the 

 resting condition. This has given rise to the doctrine of the continuity 

 of chromosomes, which supposes that each chromosome is a permanent 

 structure, which, though changing its form, is present as an individual 

 unit even during the period when the nucleus is not dividing. The proof 

 of this, however, is exceedingly difficult to obtain, and it must be regarded 

 at present as little more than a plausible theory. 



The chromosomes themselves are not homogeneous bodies, but consist 



