1 8 THE STRUCTURE OF CELLS 



skein is produced. The linin framework does not necessarily form 

 a continuous thread. Often it is more or less broken, and it almost 

 always shows cross-anastomoses (from which, however, in the later 

 phases, the chromatin is commonly absent) with the neighbouring 

 threads. This anastomosis is doubtless the expression of its segrega- 

 tion, due to contraction ; the anastomoses themselves representing 

 the original meshes by which the substance was formerly bound 

 together into a coherent whole. Simultaneously the chromatin 

 increases largely both in amount and in the intensity of its stain- 

 ing power a fact which may be taken to indicate a chemical or 

 physical change in its state. The linin thread-work that contains 

 the chromatin is often not scattered irregularly through the nucleus, 

 but is more or less polarised, as was clearly observed by Rabl, in 

 such a Avay as to converge, often with considerable distinctness, 

 towards one point on the nucleus. This point is occupied by the 

 centrosomes when they are present. At first usually lying in 

 pairs, and often in a mass of archoplasm, these bodies in the simpler 

 cases now commence to diverge, and each is either accompanied by 

 a portion of the original archoplasm, or else the latter is differenti- 

 ated progressively and afresh as they move apart to take up 

 diametrically opposite positions on the periphery of the nucleus. 

 From them radiate outwards into the protoplasm the well-known 

 astral figures which are characteristic structures in the cell at this 

 period, and are commonly regarded as of archoplasmic origin. 



Meantime within the nucleus the chromatic thread thickens and 

 shortens. Some of its substance is probably derived, at least in many 

 cases, from the nucleolus, which becomes vacuolated and often 

 fragments about this stage. Finally, the thread breaks up into a 

 number of segments which is constant for the somatic cells of the 

 species. These segments are the Chromosomes (Waldeyer). At or 

 immediately following this stage a fibrillar structure begins to 

 appear within the nucleus, and as it increases the chromosomes are 

 gradually driven to occupy an equatorial position (the equatorial 

 plate stage) in the nucleus. What is precisely to be looked on as 

 the origin of these fibrils (the so-called achromatic fibres which 

 together form the achromatic spindle) is not certainly known. 

 Some, with Strasburger, hold that they are exclusively of cytoplasmic 

 (kinoplasmic) origin, growing inwards, as it were, from the polar 

 centrospheres. Others again look on them as derived from nuclear 

 substance, whilst a third view regards them as of mixed origin. 

 Probably the last view is less open to objection than the other two. 

 The fibrillar structures themselves are almost certainly the result 

 of conditions of stress and strain in the viscous substances of which 

 the cell is composed, and it would appear probable that any sub- 

 stance capable of assuming the fibrous character might be compelled 

 to do so. And there is abundant evidence to show that such sub- 



