CELL-DIVISION 



the chromosomes on a set of "fibres," the mitotic spindle, 

 beginning with the break-down of the resting nucleus and 

 ending with the formation of two resting nuclei. If from 

 the poles of the spindle where the "fibres" more or less 

 converge, other "fibres," called astral fibres, extend radi- 

 ally into the cytoplasm, the spindle is said to be of the astral 

 type; if these radiations are absent, the spindle is called 

 anastral. In the former type at the centre of the aster may 

 be found either a single granule, the centriole or centro- 

 some, or many finer granules; or, the astral centre may be 

 of such fine structure that it appears to be optically empty. 

 Mitotic division of the nucleus may normally ensue without 

 concurrent division of the cytoplasm; experimentally it is 

 easily possible to suppress or arrest cytoplasmic division 

 while nuclear divisions continue. Since this is true, the 

 failure of cytoplasmic division after amitotic nuclear divi- 

 sion that is sometimes observed, does not deserve the 

 emphasis which some writers place upon it. The mitotic 

 complex may arise wholly or in part within or outside of 

 the nucleus. 



What should we demand of any theory of cell-division ^ 

 An explanation of cell-division and not of nuclear phe- 

 nomena. The theory should cover amitosis and mitosis.^ 

 It should apply with equal force to all types of mitosis 

 among both Protista and multicellular organisms; to every 

 variation of the mitotic configuration. I.e., to spindles with 

 and without asters, with and without centrosomes. Since 

 cytoplasmic division does not always synchronize with 

 either direct or indirect division of the nucleus, a theory of 

 cell-division based exclusively or too strongly on nuclear 

 division embraces only cells exhibiting synchrony in division 

 of cytoplasm and nucleus; not only does such a theory fail 



^ Delage, i8g^, p. /§S: " Ce qui est essentiel dans la division indi- 

 recte, c^est la division directe et cette derniere seule est a expliquer." 



