364 AN INTRODUCTION TO ZOOLOGY 



radiating lines, the astral rays, and is termed an aster. During this 

 time the chromatin of the nucleus becomes more in amount and 

 stains more readily, and it takes up its position on the linin threads 

 apparently as a series of granules. The karyosome breaks up and 

 contributes its chromatin to the general supply, while the nucleolus 

 is apparently passed out into the cytoplasm, where it disappears, 

 taking no part in the subsequent changes. All the chromatin 

 becomes arranged in a long convoluted and seemingly continuous 

 thread, termed the skein or spireme. The asters separate more 

 widely, and as they do so their adjoining fibres unite to form a long 

 spindle-shaped arrangement termed the spindle. The centrosomes 

 pass on until they come to lie at opposite poles of the nucleus, 

 through which the spindle fibres are enabled to pass because of the 

 disappearance of the nuclear membrane. Meanwhile, the spireme 

 becomes broken up into a number of definite independent pieces, 

 usually about equal in length, termed the chromosomes, which 

 finally take up a fairly symmetrical position in the middle of the 

 spindle, with their length at right angles to the line joining the 

 centrosomes, and form what is termed the equatorial plate. Thus is 

 produced a very characteristic arrangement ; there is a centrosome 

 at each end with its radiating astral rays, and, joining them, the 

 spindle, now strongly marked and quite wide in its middle region. 

 Across the widest part is arranged the equatorial plate of chromo- 

 somes, which are situated in the peripheral region of the spindle. 

 The outermost spindle fibres seem to be attached to the chromo- 

 somes, and so form, as it were, an outer zone, sometimes described 

 as the mantle fibres, surrounding the inner central core of the spindle, 

 the fibres of which pass from one end to the other. This completes 

 the typical prophase, and the resulting figure is a very striking one 

 and known as the amphiaster or achromatic figure. 



While a certain amount of variation is met with, for example, 

 the entire spindle may be well formed outside the nucleus, and later, 

 as it were, break across it ; yet we find in general a fairly close 

 similarity in the prophase, as it is found in practically all species of 

 higher animals. Among some of the Protozoa we can see certain 

 marked differences, so much so that they suggest that the process of 

 mitosis was evolved in that phylum, and that certain of its members 

 have not yet developed to the stage when its mechanism has been 

 perfected, and in simple cases the whole figure may arise within the 

 nuclear membrane. In the normal divisions of all the cells of the 

 body and its various tissues, in both the embryonic and adult stages, 

 there is produced an amphiaster. As has been pointed out, it 

 appears as if actual fibres are produced, some of which are attached 

 to the chromosomes, and others not, and it may be significant that 



