462 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Fig. 221. Stages In the maturation of the ovum; diagrammatic (mainly from Wilson) : A, the orig- 

 inal ovarian ovum; ». its nucleus, containing fbur chromosomes ; c, its double centrosome, surrounded 

 by the attraction Bphere; in /.' much of the chromatin has begun to degenerate; the rest has become 

 arranged Into two quadrupli ■ chromosomes or tetrads; the formation of the spindle and the 



asters has begun; in Cthe flrsl polar amphlaater, bearing the chromosomes, is completed ; in Dtheam- 

 phiaster has become rotated and has travelled toward the Burface of the ovum; g. v, the degenerated 

 remains of the nucleus; in E the division of the tetrads into double groups of chromosomes, or dyads, 

 has begun, and the first polar body, p. 6«, is indicated ; In J" the first polar body, containing two dyads, has 

 been extruded ; the formation ofthe Becond polar amphlaster has begun ; In G the firsi polar body is pre- 

 paring to divide . polar amphlaster is fully funned ; in // the division ofthe dyads into single 



chromosomes In both the Brsl and the egg has begun, and the Becond polar body, p. 6», is in- 



dicated : in / the formation of the [Hilar bodies is completed ; j . the egg-nucleus, containing two small 

 chromosomes, one-half the original Dumber. In fertilization the spermatozoon will bring in two addi- 

 tional chromosomes, thus restoring the total number Oi lour. 



