8 THE BREEDING OF ANIMALS 



9. How cells divide. All cells do not divide in the 

 same manner, but the most typical process is known as 

 indirect division or mitosis, and this will be here described. 

 We have seen that the nucleus contains within its minute 

 compass the active material which stimulates the cell 

 to various activities and determines its physiological 

 destiny. If further evidence was needed on this point, 

 it would be found in the remarkable and interesting trans- 

 formations which take place within the nucleus before 

 and during the process of cell division. 



The cell first passes through a vegetative or quiescent 

 stage*, and this is followed by a period of activity finally 

 resulting in the formation of two cells from the original 

 parent cell. Various clearly marked stages or phases 

 are distinguishable in this process which have been accu- 

 rately described by Wilson. The phases observed are 

 for convenience named : (1) prophase, (2) metaphase, 

 (3) anaphase, and (4) telophase. 



10. Prophase (Fig. 1). In the vegetative stage the 

 chromatin of the nucleus exists in the form of a network. 

 Generally during the prophase the chromatin loses its 

 net-like arrangement and assumes the form of a skein 

 like thread known as the spireme. During this stage 

 the spireme thread stains intensely and is fine and closely 

 convoluted. It gradually becomes thicker and the con- 

 volutions become more open, giving rise to the " open 

 spireme." Gradually the spireme breaks up into a num- 

 ber of definite straight or curved rods known as chromo- 

 somes. It is usual for the wall of the nucleus to disappear 

 during this phase, and the chromosomes then lie naked 

 in the protoplasm of the cell. It is a significant fact that 

 every plant and animal possesses a characteristic number 

 of chromosomes and that this number is always even. 



