7^ 



CELL-DIVISION 



phases the astral rays often cross one another in the equatorial region 

 outside the spindle. During the anaphases, however, this crossing 

 disappears, the rays from the two asters now meeting at an angle 

 along the cytoplasmic plate (Fig. 31). Constriction and division of 

 the cell then occur.^ 



The aster may in some cases entirely disappear, together with the 

 centrosome (as occurs in the mature dgg). In a large number of 

 cases, however, the centrosome persists, lying either outside or more 

 rarely inside the nucleus and dividing into two at a very early period. 

 This'division is clearly a precocious preparation for the ensuing divi- 

 sion of the daughter-cell, and it is a remarkable fact that it occurs as 

 a rule during the early anaphase, before the mother-cell itself has 

 divided. There are apparently, however, some cases in which the 

 centrosome remains undivided during the resting stage and only 

 divides as the process of mitosis begins. 



Like the centrosome, the aster or its central portion may persist in 

 a more or less modified form throughout the resting state of the cell, 

 forming a structure generally known as the attraction-sphere. This 

 body often shows a true astral structure with radiating fibres (Figs. 

 8, 49); but it is sometimes reduced to a regular spherical mass which 

 may represent only a portion of the original aster (Fig. 7). 



B. Origin of the Mitotic Figure 



The nature and source of the material from which the mitotic 

 figure arises form a problem that has been almost continuously under 

 discussion since the first discovery of mitosis, and is even now but 

 partially solved. The discussion relates, however, almost solely to 

 the achromatic figure (centrosome, spindle, and asters) ; for every one 

 is agreed that the chromatic figure (chromosomes) is directly derived 

 from the chromatin-network, as described above, so that there is no 

 breach in the continuity of the chromatin from one cell-generation to 

 another. With the achromatic figure the case is widely different. 

 The material of the spindle and asters must be derived from the 

 nucleus, from the cytoplasm, or from both ; and most of the earlier 

 research was devoted to an endeavour to decide between these 

 possibilities. The earhest observers {'71-7S) supposed the achro- 

 matic figure to disappear entirely at the close of cell-division, and 

 most of them (Butschli, Strasburger, Van Beneden, '75) believed 

 it to be re-formed at each succeeding division out of the nuclear 

 substance. The entire mitotic figure was thus conceived as a 

 metamorphosed nucleus. Later researches ('75- 85) gave contradic- 



i See p. 318. Cf. Kostanecki, '97, and Hoffmann, '98. 



