758 A TEXTBOOK OF PHYSIOLOGY 



daughter nuclei. The chromatin is the nuclear material, which stains 

 deeply with basic dyes, and is probably nuclein; to nucleic acid is 

 due the affinity for basic dyes. 



The series of changes in the chromatin are as follows (Fig. 461 ): The 

 granules of chromatin of the resting nucleus (a) become assembled 

 to form a skein, or spireme (6). The nucleus now elongates, its 

 membrane generally disappears, and the spireme breaks down into 

 a number of linear fragments, or chromosomes (c), the number of 

 which is constant for the nucleus of any given species. Thus, in 

 Ascaris megalocephala the number is four; in man the number is 

 sixteen. These chromosomes now assemble in the plane through 

 which the cell will divide, forming what is sometimes known as the 

 equatorial plate. The rods become bent in the fashion of a V, often 





FIG. 463. LINES OF STRESS IN A MAGNETIC FIELD. (Verworn, after Ehumbler.) 



with the angles of the V towards each other, thus forming a radiating 

 figure (d). These progressive changes are known as the prophase. 

 The chromosomes now divide longitudinally to form twice the number 

 of V's, or daughter chromosomes (e) a change termed the metaphase. 

 These chromosomes become assembled in two sets opposite each other 

 on either side of the plane of division, with the angles of the V 

 directed away from each other (/). Lastly, these two sets of 

 chromosomes blend again to form a spireme, acquire a nuclear 

 membrane (g, h), and finally break down to form once more 

 the granular masses of chromatin characteristic of the resting 

 nucleus ('). These final or regressive changes are known as the 

 anaphase. 



Frequently, the non-staining portions of the nucleus form what 

 are called the achromatic figures of mitosis. This material forms a 

 spii.dle of delicate fibrils lying at right angles to the plane along which 



