THE ACHROMATIC FIGURE 



21 





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however, that they only form a special case of karyosomes present in 

 varying numbers, and often not traceable into the prophase chromosomes. 



(5) The Achromatic Figure 



This is the name given to the centrosome and system of radiating 

 Unes proceeding from it through the cytoplasm, which are plainly con- 

 cerned with the separation of the daughter chromosomes. The term 

 refers to the fact that (with the exception of the centrosome and centriole) 

 the substance of which the system is composed (sometimes known as the 

 archoplasm) has much the same weak staining reaction as the bulk of the 

 cytoplasm and the Hnin. The main features of its development during 

 mitosis and its general disposition have already been described (Fig. 2). 



A complete achromatic figure at the metaphase of mitosis consists of 

 the following parts : (i) A minute deeply staining centrosome occupies 

 the centre of the radia- 

 tions at each pole of the 

 figure, and may contain 

 (2) a still smaller central 

 granule, the centriole. 

 The centrosome is the 

 point of insertion ^ of the 

 so-called fibres of the 

 achromatic figure, 

 namely, (3) the radiating 

 fibres composing the aster, 

 and the spindle fibres. 

 The latter are of two kinds, (4) the mantle fibres, which are attached to 

 the chromosomes, and (5) the fibres of the central spindle, which run right 

 through from one centrosome to the other. In many forms, however, a 

 central spindle seems to be absent. 



The terminology of these various parts is unfortunately in some 

 confusion, especially so far as concerns the centrosome and immediately 

 associated structures. This is largely because the centres of the system 

 are occupied by a substance arranged in concentric layers, and opinions 

 differ as to how much of this should be called centrosome. The centriole 

 is very often, indeed generally, indistinguishable from the rest of the 

 centrosome, and in this book the latter term is used to cover the centro- 

 some together with the contained centriole (when such is present) . 



Fig. 2 illustrates a case where the whole, or nearly the whole, achro- 

 matic figure arises from the cytoplasm outside the nucleus. Very often, 

 however, the spindle at least is of intranuclear origin, probably derived 



1 The fibres cannot always be traced actually into the centrosorae, but sometimes end in 

 a clear spherical mass of cytoplasm, called the centrosphere, surrounding the centrosome. 



A B 



Fig. 10. 



Prochromosomes in Pollen Mother cells of Calycanthus floridus. 

 (Overton, J.w.B., 1906.) A, resting nucleus ; B, prophase. 



