ORIGIN OF THE MIT OTIC FIGURE 53 



their division (Fig. 23). In other cases, as in the cells of the testis, 

 the remains of the spindle in each cell sometimes gives rise to a more 

 or less definite body known as \btparanucleus or Nebenkern (Fig. 62). 



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

 centrosome (as occurs in the mature egg). 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, however, some undoubted cases (cf. Figs. 6, 7) 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. 7, 

 35); but it is sometimes reduced to a regular spherical mass which 

 may represent only the centrosphere of the original aster (Fig. 6). 



B. ORIGIN OF THE MITOTIC FIGURE 



The chromatic figure (chromosomes) is derived directly from the 

 chromatic network of the resting-nucleus as described above. The 

 derivation of the achromatic figure (spindle and asters) is a far more 

 difficult question, which is still to some extent involved in doubt. By 

 the earlier observers (1873-75) the achromatic figure was supposed 

 to disappear entirely at the close of cell-division, and most of them 

 (Butschli, Strasburger, Van Beneden, '75) believed it to be reformed 

 at each succeeding division out of the nuclear substance. Later re- 

 searches (1875-85) gave contradictory and apparently irreconcilable 

 results. Fol ('79) derived the spindle from the nuclear material, 

 the asters from the cytoplasm. Strasburger ('80) asserted that the 

 entire achromatic figure arose from the cytoplasm. Flemming ('82) 

 was in doubt, and regarded the question of nuclear or cytoplasmic 

 origin as one of minor importance, yet on the whole inclined to the 

 opinion that the achromatic figure arose inside the nucleus. 1 In 1887 

 a new face was put on the whole question through the independent 

 discovery by Van Beneden and Boveri that the centrosome does not 

 disappear at the close of mitosis, but remains as a distinct cell-organ 

 lying beside the nucleus in the cytoplasm. These investigators agreed 

 that the amphiaster is formed under the influence of the centrosome, 



1 ZcUsubstanz, p. 226. 



