8 NORMAL HISTOLOGY. 



ance by which the chromatin segments are directed to the new nuclei in 

 process of formation, each daughter cell being insured in this manner one- 

 half of the maternal chromatin. 



The Prophases, or preparatory stages, include a series of changes 

 which involve the nuclear substances and the centrosome, and result in the 

 production of the mitotic figure. The latter consists of two parts, ( i ) the 

 deeply staining chromatin filaments and (2) the achromatic figure, which 

 colors only very slightly if at all. The chromatin, which before division 

 begins is disposed along the irregular nuclear fibrils, loses its reticular 

 arrangement and, increasing in amount as well as in its staining affinities, 

 becomes transformed into a closely convoluted thread or threads, constitu- 

 ting the "close skein. " The filaments composing the latter soon shorten and 

 thicken to form the ' ' loose skein. ' ' The skein, or spireme, may consist of a 

 single continuous filament, or it may be formed of a number of separate 

 threads. Sooner or later the skein breaks up transversely into a number of 

 segments or chromosomes, which appear as deeply stained curved or straight 

 rods. A very important, as well as remarkable, fact regarding the chromo- 

 somes, is their numerical constancy, since the cells of every animal and plant 

 always possess a definite number of chromosomes, corresponding to the 

 quota for that particular species; further, in all the higher animals the 

 number is even, in man being probably twenty-four. During these changes 

 affecting the chromatin, the nucleolus, or plasmosome, disappears and, 

 probably, takes no active part in mitosis. The nuclear membrane likewise 

 fades away during the prophases, the chromatic segments now lying 

 unenclosed within the cell, in which the cytoplasm and nuclear matrix are 

 continuous. 



Coincident with the foregoing changes, the centrosome, which by this 

 time has become double, is closely associated with the achromatic figure. 

 A delicate radial striation appears around each centrosome, thereby pro- 

 ducing an arrangement resembling stars or asters. The centrosomes early 

 manifest a disposition to separate towards opposite poles of the cell, this 

 -migration resulting in a corresponding migration of the asters. In conse- 

 quence of these changes, the retreating centrosomes become the foci of two 

 systems of radial striation which meet and together form an achromatic 

 figure known as the amphiaster. The latter consists of the two asters and 

 the intervening spindle. There seems little doubt that the centrosomes play 

 an important role in establishing foci towards which the chromosomes for the 

 new nuclei become attracted. Subsequently the nuclear spindle, which 

 originates from the amphiaster, often occupies the periphery of the nucleus, 

 whose limiting membrane by this time has disappeared. The delicate 

 threads of linin composing the nuclear spindle extend within an area, the 

 polar field, around which the chromosomes become grouped. The chromo- 

 somes, which meanwhile have arisen by transverse division of the chromatin 

 threads composing the loose skein, appear as V-shaped segments, the 

 closed ends of the loops being directed towards the polar field which they 

 encircle. 



The Metaphase includes the most important detail of mitosis namely, 

 the longitudinal cleavage of the chromosomes, whereby the number of the 

 latter is doubled and the chromatin is equally divided. The cleavage and 

 division are the first steps towards the actual apportionment of the chroma- 

 tin between the new nuclei, each of which receives not only exactly one-half 

 of the chromatin, but the full quota of chromosomes, and this irrespective of 

 even marked inequality in the size of the new daughter cells. 



