74 



CELL-DIVISION 



opinion that the question of nuclear or cytoplasmic origin was one of 

 minor importance. A long series of later researches on both plants 

 and animals has fully sustained this opinion, showing that the origin 

 of the achromatic figure does in fact differ in different cases. Thus 

 in Infusoria the entire mitotic figure is of intranuclear origin (there 

 are, however, no asters); in echinoderm eggs the spindle is of nuclear, 

 the asters of cytoplasmic, origin ; in the testis-cells and some tissue- 

 cells of the salamander, a complete amphiaster is first formed in the 

 cytoplasm, but to this are afterward added elements probably derived 

 from the linin-network ; while in higher plants there is some reason 

 to believe that the entire achromatic figure may be of cytoplasmic 

 origin. Such differences need not surprise us when we reflect that 

 the achromatic part of the nucleus (linin-network, etc.) is probably of 

 the same general nature as the cytoplasm. ^ 



Many observers have maintained that the material of the astral 

 rays and spindle-fibres is directly derived from the substance of the 

 protoplasmic meshwork, whether nuclear, cytoplasmic, or both; but 

 its precise origin has long been a subject of debate. This question, 

 critically considered in Chapter VI., will be here only briefly sketched. 

 By Klein i^-j^), Van Beneden ('83), Carnoy ('84, '85), and a large num- 

 ber of later observers, the achromatic fibres, both of spindles and of 

 asters, are regarded as identical with those of a preexisting reticulum 

 which have merely assumed a radiating arrangement about the cen- 

 trosome. The amphiaster has, therefore, no independent existence, 

 but is merely an image, as it were, somewhat like the bipolar figure 

 arising when iron filings are strewn in the field of a horseshoe magnet. 

 Bp3:£ri^on the other hand, who has a small but increasing following, 

 maintains that the amphiastral fibres are not identical with those of 

 the preexisting meshwork, but a new formation which, as it were, 

 "crystallizes anew " out of the general protoplasmic substance. The 

 amphiaster is therefore a new and independent structure, arising in, 

 or indirectly from, the preexisting material, but not by a direct mor- 

 phological transformation of that material. This view, which has 

 been advocated by Druner ('94), Braus ('95), Meves ('97, 4, '98), 

 and with which my own later observations ('99) also agree, is more 

 fully discussed at page 3 1 8. 



In 1887 an important forward step was taken through the inde- 

 pendent discovery by Van Beneden and Boveri that in the egg of 

 Ascaris the centrosome does not disappear at the close of mitosis, but 

 remains as a distinct cell-organ lying beside the nucleus in the cyto- 



1 In the case of echinoderm eggs, I have found reason ('95, 2) for the conclusion that the 

 spindle- fibres are derived not merely from the linin-sul^stance. but also from the chromatin. 

 Despite some adverse criticism, I have found no reason to change my opinion on this point. 

 The possible signiticance of such a derivation is indicated elsewhere (p. 302). 



