78 



CELL-DIVISION 



features. These are : (i) The formation of the chromatic and achro- 

 matic figures; (2) the longitudinal splitting of the chromosomes or 

 spireme-thread ; (3) the transportal of the chromatin-halves to the 

 respective daughter-cells. Each of these three events is endlessly 

 varied in detail ; yet the essential phenomena are everywhere the same, 

 with one important exception relating to the division of the chromo- 

 somes that occurs in the maturation of certain eggs and spermatozoa.^ 

 It maybe stated further that the study of mitosis in some of the lower 

 forms (Protozoa) gives reason to believe that the asters are of second- 

 ary importance as compared with the spindle, and that the formation 

 of spireme and chromosomes is but tributary to the division of the 

 smaller chromatin-masses of which they are made up. 



I. J'aricties of the Mitotic Figure 



(a) TJic Achromatic Figure. The phenomena involved in the his- 

 tory of the achromatic figure are in general most clearly displayed 

 in embryonic or rapidly dividing cells, especially in egg-cells (Figs. 

 31, 60), where the asters attain an enormous development, and the 

 centrosomes are especially distinct. In adult tissue-cells the asters 

 are relatively small and difficult of demonstration, the spindle large 

 and distinct ; and this is particularly striking in the cells of higher 

 plants where the asters are but imperfectly developed. Plant-mitoses 

 are characterized by the prominence of the cell-plate (Fig. 34), which 

 is rudimentary or often wanting in animals, a fact correlated no 

 doubt with the greater development of the cell-membrane in plants. 

 With this again is correlated the fact that division of the cell-body in 

 animal cells generally takes place by constriction in the equatorial 

 plane of the spindle ; while in plant-cells the cell is usually cut in 

 two by a cell-wall developed in the substance of the protoplasm and 

 derived in large part from the cell-plate. 



In animal cells we may distinguish two general types in the forma- 

 tion of the amphiaster, which are, however, connected by interme- 

 diate gradations. In the first of these, typically illustrated by the 

 division of epithelial and testis-cells in the salamander (Flemming, 

 Hermann, Drijner, Meves), a complete amphiaster is first formed in 

 the cytoplasm outside the nucleus, while the nuclear membrane is 

 still intact. As the latter fades away and the chromosomes appear, 

 some of the astral rays grow into the nuclear space and become 

 attached to the chromosomes, which finally arrange themselves in a 

 ring about the original spindle (Figs. 27, 28). In the completed 

 amphiaster, therefore, we may distinguish the original central spi7idle 

 (Hermann, '91) from the surrounding mantle-fibres, the latter being 



1 Cf. Chapter V. 



