ORIGIN OF THE INDIVIDUAL 



225 



to remember that variations in the details are as numerous as 

 the different types of cells, and that any general account can 

 do no more than present the fundamental plan of operations. 



FIG. 123. Diagrams of typical stages in mitosis. A, resting cell with chromatin 

 presenting a net-like arrangement within the nuclear membrane; c, centrosome divided; 

 B, Prophasc (early): centrosomes, asters (a), and spindle; most of the chromatin 

 material seems to assume the form of a long thread (spireme); C, prophase (later) 

 involving the disappearance of the nuclear membrane, and the separation of the chro- 

 matin of the spireme stage into discrete bodies (chromosomes); D, prophase (final) 

 with chromosomes arranged in the equatorial plate (ep); E, metaphase; each chromo- 

 some splitting lengthwise; F, anaphase: the daughter sets of chromosomes moving 

 toward the asters; if, 'inter-zonal fibers'; G, H, early and later telophase involving the 

 gradual loss of visibility of chromosomes as they spin out into the resting net-like 

 arrangement of the chromatin; division of the cytoplasm; n, nucleolus. (After Wilson.) 



Broadly speaking, mitosis can be divided into four chief 

 stages: PROPHASE, METAPHASE, ANAPHASE, and TELOPHASE, 

 during each of which characteristic changes take place in 

 the nucleus, cytoplasm, and centrosome. (Figs. 8, 123.) 



