15° 



BIOLOGY OF THE LABORATORY MOUSE 



Small liquid filled cavities appear among the cells of the cumulus oophorus 

 and the granulosa cells which line the antrum. These gradually detach the 

 cumulus oophorus from the surrounding cells so that it floats free in the 

 antrum (Fig. 77). In the meantime, in the nucleus of the ovum which lies 

 near the surface of the cell, the nuclear membrane becomes faint, irregular 

 and gradually disappears. The nucleolus also disappears, and the chroma- 



mm- 





I 



•i ■ 



Germinal epithelium 



Tunica albuginea 



Cumulus oophorus 



Membrana granulosa 





-Theca interna 



Liquor foUiculi 



Theca externa 



Blood vessel 



tin granules form small dense chromosomes (20 tetrads). Delicate achroma- 

 tic fibers form a rather narrow spindle and the chromosomes collect at the 

 equatorial plane. There are no centrosomes or astral radiations. As the 

 chromosomes begin to move toward the poles, 20 diads and a small amount 

 of the cytoplasm are separated from the ovum by constriction and the first 

 polar body is formed. It lies within the zona pellucida causing a bulging 

 of its surface. The nuclear material of the ovum does not return to a resting 

 condition. Instead preparations begin at once for the next division. 



