MITOTIC DIVISION. 13 



doubled and the chromatin is equally divided. This division is the first step towards 

 the actual apportionment of the chromatin between the new nuclei, each of which 

 receives exactly one-half of the chromatin, irrespective of even marked inequality 

 in the size of the daughter cells. 



Meanwhile the centrosomes have continued to separate towards the opposite 

 poles of the cell, where, surrounded by their attraction spheres, each forms the 

 centre of the astral striation that marks either pole of the amphiaster, the nuclear 

 spindle being formed by the junction of the prolonged and opposing striae. The 

 purpose of the achromatic figure is to guide the longitudinally divided chromosomes 

 towards the new nuclei during the succeeding changes. 



The Anaphases accomplish the migration of the chromosomes, each pair of 

 sister segments contributing a unit to each of the two groups of chromosomes that 

 are passing towards the poles of the achromatic spindle ; in this manner each new 

 nucleus receives not only one-half of the chromatin of the mother nucleus, but also 

 the same number of chromosomes that originally existed within the mother cell, the 

 numerical constancy of the particular species being thus maintained. 



Anticipating their passage towards the poles of the achromatic figure, the mi- 

 grating chromatic segments, attracted by the linin threads, for a time form a com- 

 pact group about the equator of the spindle known as the equatorial plate. As the 

 receding segments pass towards their respective poles, the opposed ends of the sep- 

 arating chromosomes are united by intervening achromatic threads, the connecting 

 fibres. Sometimes the latter exhibit a linear series of thickenings known as the 

 cell-plate or viid-body. The migration of the chromosomes establishes the essential 

 features of the division of the nucleus, since the subsequent changes are only repe- 

 titions, in inverse order, of the changes already noted. 



The Telophases, in addition to the final stages in the rearrangement of the 

 chromatic segments of the new nuclei, including the appearance of the daughter 

 wreath, the daughter skeins, the new nuclear membrane, and the nucleolus, witness 

 the participation of the cytoplasm in the formation of the new cells. In these final 

 stages of mitosis the cell-body becomes constricted and then divides into two, the 

 plane of division passing through the equator of the nuclear spindle. Each of the 

 resulting masses of cytoplasm invests a new nucleus and receives one-half of the 

 achromatic figure consisting of a half-spindle and one of the asters with a centro- 

 some. The new cell, now possessing all the constituents of the parent element, 

 usually acquires the morphological characteristics of its ancestor and passes into a 

 condition of comparative rest until called upon, in its turn, to enter upon the com- 

 plicated cycle of mitosis. 



MITOTIC DIVISION. 



I. Prophases. 



A. Changes withiyi the nucleus : Chromatic figure. 



Chromatin loses reticular arrangement, 



Close skein. 



Loose skein. 



Disappearance of nucleolus, 



Division of skein into chromosomes. 



Arrangement around polar field — mother wreath, 



Disappearance of nuclear membrane. 



B. Changes within the cytoplasm : Achromatic figure. 



Division of centrosome. 



Appearance of asters. 



Migration of centrosomes. 



Appearance of spindle, 



Formation of amphiaster, 



Appearance of nuclear spindle and polar field. 



II. Metaphase. 



Longitudinal cleavage of chromosomes. 



