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GENERAL EMBRYOLOGY 



H 



FIG. 78. Diagram of reduction, with tetrad formation, in oogenesis. From 

 "Wilson, "Cell." The somatic number of chromosomes is supposed to be four. 

 A. Initial phase; two tetrads have been formed in the germinal vesicle. B. The 

 two tetrads have been drawn up about the spindle to form the equatorial plate 

 of the first polar mitotic figure. C. The mitotic figure has rotated into position, 

 leaving the remains of the germinal vesicle at g.v. D. Formation of the first 

 polar body; each tetrad divides into two dyads. E. First polar body formed; 

 two dyads in it and in the egg. F. Preparation for the second division. G. 

 Second polar body forming and the first dividing; each dyad divides into two 

 single chromosomes. H. Final result; three polar bodies and the egg-nucleus 

 (9), each containing two single chromosomes (half the somatic number); c, the 

 egg-centrosome which now degenerates and is lost. 



