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TEXT-BOOK OF EMBRYOLOGY. 



tetrad separates into two dyads, and one dyad from each tetrad passes 

 into a small mass of cytoplasm which becomes detached from the egg cell as 

 the first polar body (Fig. 10, F, G). A new spindle forms without the return 

 of the nucleus to the resting stage, and each dyad divides into two monads. 

 The second polar body is now given off in the same manner as the first 

 (Fig. 10, H, 7, /, K). One monad from each dyad passes into a small mass 

 of cytoplasm and is separated from the egg cell. The maturation is now 

 complete. The nucleus of the mature ovum contains the haploid number 

 of chromosomes and is ready for union with the male pronucleus. 



D 10 F 



FIG. n. From sections of ova of the mouse, showing stages in the maturation process. Sobotta. 



A, Ovum showing prophase of maturation division. /, fat; z.p., zona pellucida. 



B, Ovum showing maturation spindle with chromatin segments undivided. 



C, Ovum showing diaster stage of maturation division, formation of ist polar body (p.b.), 



and sperm nucleus (male pronucleus, m.pn.) just after its entrance. 



D, Ovum showing polar body (p.b.) and male (m.pn.) and female (f.pn.) pronuclei. 



E, Ovum showing both polar bodies (p.b.} and pronuclei. 



F, Ovum showing pronuclei preparing to unite. 



The maturation of the mouse ovum, described by Mark and Long, may 

 be taken as an example of mammalian maturation. The diploid number of 

 chromosomes is twenty, but when the growth of the primary oocyte is 

 completed and the cell prepares for division only ten chromosomes are 

 present. Each chromosome is V-shaped and shows the structure of a tetrad. 

 While still in the Graafian follicle the first polar body is given off and lies 

 as a small globule beneath the zona pellucida. The egg cell and the first 



