502 EVOLUTION, HEREDITY, EUGENICS 



somes develop from the nuclear reticulum without the formation of 

 a spireme. 



The centrosomes move apart and about each centrosphere appears 

 a group of radiating lines called the aster. Between the centro- 

 spheres other lines form the spindle. The spindle and asters, stain- 

 ing lightly, are called the achromatic figure, or arnphiaster. 



The chromatin thread {spireme) breaks into segments called 

 chromosomes^ which group themselves in the equator of the spindle, 

 forming the chromatic figure called the equatorial plate. ^ 



II. Metaphase. — The chromosomes which have split lengthwise 

 now separate into two exactly similar groups. As we shall see later 

 this equal distribution of chromatin has great significance. 



///. Anaphase. — As the chromosomes divide, the daughter 

 chromosomes draw apart and diverge in two groups to opposite 

 poles of the spindle. The two masses are connected by a bundle 

 of achromatic fibers, called intej-zonal connecting fibers. In the later 

 anaphase stages, the cell membrane shows signs of the constriction 

 which will later produce two daughter cells. 



IV. Telophase. — The entire cell divides into two by a plane 

 passing through the equator of the spindle. This is often indicated 

 before division takes place by a peculiar modification of the cyto- 

 plasm in the equatorial plane outside the spindle. 



Each daughter cell receives a group of chromosomes, half of the 

 spindle, and one aster with its centrosome. The daughter chromo- 

 somes become thickened, and form a daughter spireme, similar to 

 that of the mother nucleus. 



Then the prophases are to some extent retraced, in inverse order, 

 the chromosomes returning from the spireme to a chromatic net- 

 work. The astral rays disappear, and the nuclear membrane 

 returns.^ 



^ Heilbrunn, studying the viscosity of protoplasm in the eggs of echinoderms 

 annelids and molluscs during mitosis, found two maxima of increase, one in the pro- 

 phase and a second j ust prior to cleavage. Chambers, using the micro-dissection needle, 

 found also that the viscosity of the aster decreases from the center towards the periph- 

 ery. (Consult L. V. Heilbrunn, The Colloid Chemistry of Protoplasm. Protoplasma 

 Monographieren, Geb. Borntraeger, Berlin, 1928, and The Viscosity of Protoplasm. 

 Quar. Rev. of Biol., vol. 2, pp. 230-248, 1927.) 



^ In the sea urchin, 7 successive divisions, giving rise to 128 cells, may occur in 3 

 hours. During the life of an individual, dividing cells are to be found at all times. 

 Jolly reported that in the salamander, the prophases took 50 minutes, the metaphase 

 but 4 minutes, the anaphase 48 minutes, and that reconstruction took but 39 minutes. 



