THE OVUM 



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single ovum, but sometimes two or more are present. 1 By the enlargement and 

 subsequent rupture of a follicle at the surface of the ovary, an ovum is liberated and 

 conveyed by the uterine tube to the cavity of the uterus. Unless it be fertilized 

 it undergoes no further development and is discharged from the uterus, but if 

 fertilization take place it is retained within the uterus and is developed into a 

 new being. 



In appearance and structure the ovum (Fig. 3) differs little from an ordinary 

 cell, but distinctive names have been applied to its several parts; thus, the cell 

 substance is known as the yolk or obplasm, the nucleus as the germinal vesicle, and 

 the nucleolus as the germinal spot. The ovum is enclosed within a thick, trans- 



parent envelope, the zona striata or zona pellucida, adhering to the outer surface 

 of which are several layers of cells, derived from those of the follicle and collectively 

 constituting the corona radiata. 



Yolk. The yolk comprises (1) the cytoplasm of the ordinary animal cell with its 

 spongioplasm and hyaloplasm; this is frequently termed the formative yolk; (2) 

 the nutritive yolk or deutoplasm, which consists of numerous rounded granules of 

 fatty and albuminoid substances imbedded in the cytoplasm. In the mammalian 

 ovum the nutritive yolk is extremely small in amount, and is of service in nourish- 



See description of the ovary on a future page. 



