374 i-IFE • OUTLINES OF GENERAL BIOLOGY 



appearance becomes very different. The skein or network disap- 

 pears, and the chromatin becomes concentrated into sharply 

 defined bodies called chromosomes, definite in number for each 

 species. Just outside the nucleus, and probably arising from it, is a 

 small body called the centrosome. This diN-ides into two, and the 

 two halves move to opposite poles of the nucleus and become th( 

 centres of what apix^ar in the fixed cytoplasm as very delicate rays, 

 .^t the equator of the nucleus the chromosomes arrange themselves 

 in a horizontal plane, at right angles to the long axis of the cell, 

 or to the line between the two centrosomes. Each chromosome is 

 then split up the middle longitudinally, as one might split a wooden 



Fig. 49. 



Diagram of Typical Cell-division. After Zicglcr. i, The resting nucleus, 

 with centrosome (C); 2, the centrosome has divided into two; 3, the 

 chromosomes are now well-defined; 4, chromosomes grouped at the 

 equator; 5 and 6, each chromosome is longitudinally split; 7, the halves 

 of the chromosomes retreat towards the poles; 8, a new cell membram- 

 is formed; 9, the daughter cells are now quite distinct and the nuclei 

 are passing back into the resting phase. 



match into two. The limits between the nucleus and the cytoplasm 

 outside become vague, and the round appearance of the nucleus 

 is replaced by a more or less biconical spindle shape. Then a remark- 

 able thing happens: one-half of each longitudinally spHt chromo- 

 some moves towards each centrosome— pushed or pulled, who shall 

 say? In any case, the half chromosomes move along the delicate 

 fibres which run from centrosome to centrosome (so forming the 

 "central spindle") or from centrosome to equator (the "equatorial 

 spindle"). In this arrangement there is a curious likeness to that of 

 iron filings round a magnet or arranging themsdves on a plate 

 under electrical influence. Then the cell constricts and separates the 

 two poles from ont; another, thus soon forming two daughter-cells, 

 each with its normal number of chromosomes, and each with a 

 centrosome. The chromosomes gradually sink back into the resting 



