3^4 



GENERAL PRINCIPLES 



this time two centrosomes make their appearance close beside 

 the nucleus. They are connected by a number of slender fibres, 

 which curve outward in the middle so that all together form 

 a spindle-shaped figure. The centrosomes gradually recede 



Fig. i8o. — Diagrams illustrating the prophases of mitosis. A, Beginning of 

 formation of the spindle. B, chromosomes formed. C, Chromosomes approach- 

 ing the equator of the spindle. D, Chromosomes ready to divide. (See next 

 figure.) 



from each other until they come to lie at opposite poles of the 

 nucleus. During these changes the nucleoli have gradually dis- 

 appeared and the nuclear membrane also becomes indistinct 

 and fades away. The spindle fibres now traverse the nucleus 



