28 MITOSIS: THE CONSTANCY OF THE CHROMOSOMES 



separation of the chromatids. This is not so. Separation always 

 begins at the centromere, at a time when the distal ends are more 

 closely united than earlier, and a wide separation of the distal ends 

 shows an early metaphase stage, not a late one [cf. Fig. 5). 



(iv) Metaphase : the Plate. While their centromeres lie on the 

 equator of the spindle, the whole group of chromosomes lies in a 

 " plate," the metaphase plate, which has been generally studied in 

 polar view in order to make out the number, sizes and shapes of the 

 chromosomes that are characteristic of the organism, the race or the 

 species. In number and size the chromosomes are, as a rule, 

 invariable at this stage throughout the organism. In shape they 

 may vary slightly owing to different degrees of spiralisation. In 

 very rapid gonial mitoses they may be less spiraUsed than usual {e.g. 

 in the protozoan Aggregata, Belar, 1926), while in the mitoses 

 immediately preceding meiosis in the Orthoptera and in that 

 immediately following meiosis in the Angiosperms they are usually 

 more spiralised (Janssens, 1924 ; White, 1935, on Mecostethus ; 

 Darlington, 1929, on Tradescantia ; Janaki-Ammal, unpub., on 

 Saccharum). 



In distribution on the plate two main types are found in higher 

 organisms. In the first, the so-called " central spindle " type of 

 Salamandra, the centromeres lie at the edge of the spindle and the 

 bodies of the chromosomes lie outside in the cytoplasm. In the 

 second, which is usual in plants, the centromeres lie evenly in the 

 equatorial plane of the spindle and the bodies of most of the chromo- 

 somes are therefore inside the spindle, fiat on the plate when they 

 are small, turned up on either side when they are longer. There 

 is no absolute distinction between these types. Many Orthoptera 

 are intermediate and abnormal pollen grains of Fritillaria and 

 Tulipa have been found with the Salamandra type of plate. Such 

 variations will probably be found to be widespread. 



(v) Anaphase. The end of metaphase is marked by a change in 

 the centromere or, where it is not visible, at the centric constriction. 

 The two chromatids which have lain closest together at this point 

 now separate (Figs. 4 and 19B). Under suitable conditions of 

 fixation the centromere becomes visible in each chromatid at the 

 point of separation (S. Nawaschin, 1912 ; Trankowsky, 1930). 



