CELL DIVISION 103 



the number of chromosomes in the somatic and germinal 

 cells, the latter having twice the number of the former. 

 For example, the somatic cells of human beings, oxen, 

 and guinea-pigs have all sixteen chromosomes, the 

 germinal cells thirty-two; the somatic cells of the mouse, 

 salamander, and trout twenty-four, the germinal cells 

 forty-eight; the somatic cells of ascarides two, their 

 germinal cells four. 



About the time that the chromosomes become dis- 

 tinct it can also be determined that each has undergone 

 a longitudinal cleavage into two parallel threads of 

 exactly equal size. 



In cells in which no centrosome is usually visible, that 

 organ now makes its appearance surrounded by a con- 

 densed or highly granular area of cytoplasm known as 

 the attraction sphere, which is to become the " polar 

 field." 



2. The Disappearance of the Nuclear Membrane and 

 the Division of the Chromosomes. During these changes 

 the nuclear membrane has imperceptibly disappeared 

 leaving the chromosomes free in the cytoplasm. The 

 chromosomes in the meantime have uniformly assumed 

 a V-shape and arranged themselves about the polar 

 field with the apices toward the centre. It now makes 

 considerable difference from which direction one views 

 the groups of chromosomes, for at the same time that 

 this adjustment has been in progress the centrosome 

 has divided and its halves have gradually separated so 

 that we have now a "polar field" and a "hypopolar 

 field, " each occupied by a centrosome from one to the 

 other of which delicate filaments of linin extend forming 

 a peculiar object known as the "nuclear spindle." 



If the group of chromosomes be viewed from a direc- 

 tion corresponding to the polar or hypo-polar fields, one 

 sees those bodies with their apices directed toward 

 the centre, their ends outward, forming an appearance 

 sometimes compared to a wreath, but usually to a star 

 and spoken of as the "mother star." If, on the other 



