CH. LIX.] 



MATURATION OF THE OVUM 



875 



promptly to the stimulus of an ovum by the formation of a decidua, 

 or as Pfliiger taught, because it provides a raw surface on which the 

 ovum is easily grafted. 



The ovum, after it is liberated from the ovary by the rupture of 

 a Graafian follicle, enters the Fallopian tube, the cilia on the 

 fimbrise of which are the main instruments for the transportation. 

 It travels along the Fallopian tube, and finally reaches the uterus, 

 and again this is accomplished by ciliary action. During this 

 journey, which probably occupies some days, it becomes mature, it 

 is fertilised, and some of the early steps in further development 

 may also occur. 



Maturation of the Ovum. We have already seen that the germ 

 cells which form the ova are at first imbedded in the germinal 



Fro. 561. Diagram showing the formation of the polar bodies (maturation of the ovum). A, B, and C 

 show stages in the formation of the first polar body by heterotype mitosis. A is a primary 

 oiicyte at the commencement of mitosis, when only half the usual number of chromosomes 

 appear. C is a secondary oiicyte ; it has no distinct nucleus, because no resting-stage occurs ; after 

 the separation of the first polar body, the chromosomes which remain in the secondary oiicyte at 

 once rearrange themselves on a new spindle. D is the mature ovum, with the female pronucleus 

 and the two polar bodies. 1, First polar bud; 2, first polar body; 8, second polar body; 4, 

 chromosomes on spindle of primary oiicyte; 5, zona striata ; 6, viielline membrane; 7, daughter 

 chromosomes in first polar bud ; 8, female pronucleus. 



epithelium, from which they pass into the stroma of the ovary, and 

 then by division and growth they form oogonia ; from the oogonia, 

 primary oocytes are developed, and these become enclosed in Graafian 

 follicles. The process by which the primary oocytes become con- 

 verted into mature ova is known as maturation; this consists 

 essentially of a double mitotic division of the oocyte, each division 

 producing two unequal parts. The first division produces a secondary 

 oocyte and the first polar body ; the second division, which takes place 



