48 CENTROSOMES, SPINDLE AND ASTERS [CH. 



With regard to the nature and origin of the astral rays 

 and spindle-fibres opinions vary no less than in respect of 

 the centrosomes. The many hypotheses that have been put 

 forward may be classed in three main groups : 



(1) A considerable number of observers maintain that 

 the asters and spindle are merely modifications of a reticular 

 or alveolar structure existing in the cytoplasm generally 

 that the threads of the reticulum or walls of the alveoli take 

 on a radial arrangement around the centrosomes, and that 

 where the rays from the two centres meet one another they 

 unite and form the spindle. 



(2) BOVERI (1888), on the contrary, formed the opinion 

 that the whole mitotic figure is formed from a special kind 

 of protoplasm, named by him archoplasm, which in the 

 resting cell exists around the centrosome and from which, 

 when nuclear division approaches, the asters and spindle are 

 produced. BOVERI'S archoplasm hypothesis has been adopted 

 in a somewhat modified form by many subsequent writers, 

 who maintain that while the fibres of the asters and spindle 

 differ in nature from the general protoplasm of the cell, yet 

 the whole substance from which the mitotic figure is formed 

 is not present in the resting cell as such; they regard the 

 fibres as growing out from the neighbourhood of the centro- 

 some and being formed from the general cytoplasm as they 

 grow, much as crystals may grow in a saturated solution. 

 In this sense the fibres would be formed by differentiation 

 from the cytoplasm, but would not be due to a merely local 

 rearrangement of its structure. 



(3) A third group of cytologists believe that the spindle 

 and asters are simply the expression of lines of force radi- 

 ating from the centrosomes, and causing the particles of 

 the cytoplasm to arrange themselves in lines or chains as 

 iron-filings do under the influence of a magnetic field. 



