890 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



FIG. 308. Stages in the maturation of the ovum ; diagrammatic (mainly from Wilson) : A, the orig- 

 inal ovarian ovum ; n, its nucleus, containing four chromosomes ; c, its double centrosome, surrounded 

 by the attraction sphere ; in B much of the chromatin has begun to degenerate ; the rest has become 

 arranged into two quadruple groups of chromosomes, or tetrads ; the formation of the spindle and the 

 asters has begun; in C the first polar amphiaster, bearing the chromosomes, is completed ; in D the am- 

 phiaster has become rotated and has travelled toward the surface 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. &*, is indicated ; in F the first polar body, containing two dyads, has 

 been extruded ; the formation of the second polar amphiaster has begun ; in G the first polar body is pre- 

 paring to divide ; the second polar amphiaster is fully formed ; in H the division of the dyads into single 

 chromosomes in both the first polar body and the egg has begun, and the second polar body, p. &*, is in- 

 dicated; in I the formation of the polar bodies is completed ; 9, the egg-nucleus, containing two small 

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

 tional chromosomes, thus restoring the total number of four. 



