34 AN INTRODUCTION TO MODERN GENETICS 



5. Behaviour of Chromosomes^ 



The chromosomes are normally visible only during cell division, 

 when they appear in stained preparations as darkly staining bodies in 

 the nucleus. In life they can be distinguished with some difficulty with 

 ordinary light, but can be photographed with good contrast by ultra- 

 violet light, which they absorb more strongly than does the cytoplasm.^ 

 There are two types of cell division to be considered. 



5fl. Mitosis; the normal type of cell division 



The nucleus of a cell in the resting stage (or interphase) is more or 

 less optically homogeneous in life, and shows only an irregular network 

 when fixed and stained (see p. 39). The first structures which appear 

 at the beginning of a division (the prophase stage) are thin threads 

 which can be seen to be double. The members of each doublet are 

 similar and lie closely side by side, but in certain organisms there may 

 be several obviously different types of doublet. The double threads 

 contract in length and become thicker, and gradually develop a definite 

 shape. By the time their shapes can be clearly recognized, it can be 

 seen that there are an even number of doublets and always two of 

 each type (with a few exceptions, see sex chromosomes, p. 75). Each 

 double body is one chromosome split longitudinally into two half- 

 chromosomes or chromatids. Thus there are two of each sort of 

 chromosome (four of each sort of chromatid), and the total number 

 of chromosomes, which is normally even, is spoken of as the diploid 

 number. 



The chromatids are held together at one point, which can be seen as 

 a non-staining gap in the chromosome (the attachment constriction or 

 centromere). It was at one time thought that a fibre of the spindle was 

 joined on to the chromosome at this point, but it is now beUeved that 

 the spindle fibres are artefacts of fixation, corresponding indeed to some 

 latent structure in the spindle cytoplasm but not actually existing as 

 definite fibres until coagulated by the fixative (p. 363). By the time the 

 contraction of the chromosomes is complete, the nuclear membrane has 

 disappeared and the spindle has been formed; there may be acrosomes 

 or centrosomes at the poles of the spindle in animals or lower plants 

 but they are absent in higher plants, where the whole spindle mechan- 

 ism is less highly developed. The chromosomes become arranged with 



^ For general accounts of t±ie cytology of the nucleus, see Belar 1928, 

 Darlington 1937, Geitler 1934, Sharp 1934, White 1937, Wilson 1928. 

 2 Lucas and Stark 193 1, Caspersson 1936. 



