MITOTIC DIVISION OF CELLS. 71 



living protoplasm really corresponds to the appearances 

 exhibited in our preparations. 



The differences in their staining reactions of course indicate 

 corresponding differences in chemical composition between the 

 chrornatin and the cytoplasm, and analysis has shown that the 

 chromatin is characterized by the presence of relatively large 

 quantities of phosphorus. This is contained in . the complex 

 nucleinic acid, with which various albuminous bodies may be 

 combined to form the chromatin substance. 



When a typical animal cell is about to divide another structure 

 makes it appearance, usually just outside but occasionally inside 

 the nucleus. This is the centrosome (Fig. 30, c.s.), a very 

 minute body which has peculiar staining properties and which 

 is surrounded by a Differentiated area of protoplasm known 

 as the centrosphere or attraction sphere (Fig. 31, A, csph). 

 It has been questioned whether a centrosome and attraction 

 sphere are always present or whether they make their 

 appearance only when the nucleus is about to undergo 

 mitosis. This process certainly seems to be initiated by the 

 centrosome, which may divide into two parts long before the 

 nucleus itself commences to do so, so that two centrosomes often 

 appear alongside the so-called resting nucleus (Figs. 30 ; 31, A). 

 Presently the two centrosomes move away from one another, 

 both still keeping close to the nucleus, and each is now seen to be 

 surrounded by its own attraction sphere (Fig. 31, B). Around 

 each attraction sphere delicate threads or fibrillse of the cyto- 

 plasm become radially arranged to form a star or aster, and 

 the rays of the asters which lie between the two centrospheres 

 combine to form a spindle (Fig. 31, C, ksp). 



In the meantime remarkable changes have commenced in the 

 nucleus itself. The chromatin granules, together with the liuiu 

 by which they are apparently held together, have arranged them- 

 selves in the form of a long coiled thread, the spirerne (Fig. 31, B), 

 and presently the nuclear membrane begins to disappear (Fig. 31, 

 C, km), being apparently dissolved in the general protoplasm. 

 In this way the distinction between cytoplasm and nucleoplasm 

 is obliterated. 



The spireme thread breaks up into a number of short 

 lengths known as chromosomes (Fig. 31, C, chrs), the actual 

 number being, with certain exceptions, a constant character for 

 each species of plant or animal. The centrosomes at about this 



