194 



EVOLUTION, GENETICS, AND EUGENICS 



CELL DIVISION — MITOSIS 



Each cell during periods of growth and development grows to a 

 definite size and then divides into two daughter cells. Repeated cell 



divisions multiply vastly the numbers 

 of cells in a developing embryo, until 

 in higher organisms millions of cells 

 are produced. To make what might 

 easily be a long story as short as pos- 

 sible, let it be sufficient to state that 

 the chief features of mitotic cell divi- 

 sion (mitosis) are as follows: The 

 central body divides, and the two cen- 

 ters migrate apart, spinning between 

 them a spindle of fibers, the mitotic 

 spindle (Fig. 41). The nuclear mem- 

 brane disappears; and, as the two 

 central bodies migrate farther and far- 

 ther apart, the chromosomes, which 

 have in the meantime gradually be- 

 come more and more compact, move 

 toward the equator of the spindle, each 

 attached by two fibers probably com- 

 posed of linin, one fiber leading to one 

 pole of the spindle, the other fiber to 

 the other pole (Fig. 42) . Usually even 

 before migrating into the spindle each 

 chromosome has become a double 

 body, a pair of joined twins, each one 

 the exact duplicate from end to end of 

 the other. Each gene in the gene-chain 

 has previously twinned, so that each 

 gene in each chromosome is a double 

 or twin gene. The chromosomes now 

 arrange themselves in a plane at right 

 angles to the long axis of the spindle, 

 each chromosome having one of its 

 twin halves directed to one pole and 

 the other to the opposite pole. All of the stages up to this point of 

 equilibrium are known as prophases of mitosis, the equilibrium phase 

 being the meta phase (Fig. 42, E). 



Fig. 41. — Diagram of the early 

 phases of mitosis in Ascaris. A, 

 vegetative nucleus; B, fine spireme; 

 C, coarse spireme; D, late prophase 

 with chromosomes, spindles form- 

 ing. {From E. B. Wilson.) 



