344 REPRODUCTION 



copulation and ending only when the offspring can fend 

 for itself. 



Division of Cells 



Division of cells occurs by a process known as mitosis 

 or karyokinesis. It involves the nucleus as well as the 

 cytoplasm. 



In the resting cell the nucleus is surrounded by an irregular 

 mass of a basophile material — chromatin. In the cyto- 

 plasm is a small star-shaped body — the centrosome. 

 Division takes place in the following stages : — 



1. The chromatin is arranged in one long skein or 

 spireme. 



2. The skein divides into a number of segments which 

 are sometimes V-shaped. These are now known as chro- 

 mosomes. The number of chromosomes is constant for 

 each species, the number in man being twenty-four. Mean- 

 while the centrosome has divided, one division migrating 

 to the opposite side of the nucleus. The stellate appear- 

 ance of the centrosomes becomes accentuated and the 

 two become joined together by fine lines in the form of a 

 spindle — the achromatic spindle {Diaster stage). 



3. The chromosomes dispose themselves radially at the 

 equator of the spindle. 



4. Each chromosome divides longitudinally, the halves 

 separating and passing to the two centrosomes. Here they 

 join up into a skein, eventually resuming the form of the 

 resting nucleus. 



5. The spindle disappears, and at its equator the cyto- 

 plasm is modified to form a partition. In this way the 

 division of the cells is completed. 



This, the usual form, is known as homotype mitosis. 



A modified form of mitosis, known as heterotype, occurs 

 at one stage in the course of formation of the mature 

 male and female sexual cells. The number of chromosomes 

 into which the skein divides is only half the normal. These 

 divide transversely instead of longitudinally. In the male, 



