14 



GERM-CELL CYCLE IN ANIMALS 



nucleolus . As we shall see later (Chapter V) , amitosis 

 has been described in cells of the germ-cell cycle, 

 and must therefore be reckoned with in any discus- 

 sion of the phys- 

 ical basis of 

 heredity. 



Mitosis or ka- 

 ryokinesis in- 

 volves a rather 

 complicated 

 series of pro- 

 cesses which 

 cannot be fully 

 discussed here 

 but will be out- 

 lined very briefly 

 with the aid of 

 Fig. 3. 



(a) During 

 the prophase 

 the chromatin 

 granules which 

 are scattered 

 through the nucleus in the resting cell (A) become 

 arranged in the form of a long thread or spireme (B) . 

 At the same time the centrosomes move apart (A, c; 

 B, a), and a spindle arises between them (C). While 

 this is going on, the nuclear membrane generally 

 disintegrates and the spireme segments into a num- 

 ber of bodies called chromosomes (C) ; these take a 

 position at the equator of the spindle, halfway be- 







Fig. 2. — Amitosis. A. Division of blood-cells 

 in the embryo chick, illustrating Remak's 

 scheme, a—e = successive stages of division. 

 {From Wilson, 1900.) B. Amitotic nuclear 

 division in the follicle cells of a cricket's egg. 

 {From Dahlgren and Kepner, 1908.) 



