704 LIFE : OUTLINES OF GENERAL BIOLOGY 



on this there lie scattered granules of a protein substance which 

 stains deeply and is therefore called chromatin. The nucleus also 

 contains a compact body called a nucleolus. Such is the resting 

 nucleus, but in the process of cell-division (the mode known as 

 "karyokinesis" or "mitosis") the appearance becomes very different. 

 The skein or network disappears, and the chromatin becomes con- 

 centrated into sharply defined bodies called chromosomes, definite 

 in number for each species. The lowest number is two, seen in the 

 cells of Ascaris megalocephala, a common Nematode parasite of the 

 horse. One of the largest numbers is 64 in the horse itself. Man's 

 number is 48. Just outside the nucleus, and probably arising from it, 



Fig. 106. 



Stereoscopic View of a Dividing Cell. C, the centrosome, surrounded by an 

 astrosphere of radiating threads; CHR, the chromosomes on the equa- 

 torial plane; SP, the central spindle of threads running from pole to 

 pole; CY, the general cytoplasm of the cell. From a model. 



is the small body called the centrosome. This divides into two, and 

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

 centres of what appear in the fixed cytoplasm as very delicate rays. 

 At 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 

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

 outside become vague, and the spherical 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 chromosome 

 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 



