i6o8 



A MANUAL OF ANATOMY 



pregnancy. (3) In extremely rare cases the ovum may not leave the Graafian 

 follicle, even though that follicle and the ovary should rupture in the usual 

 way. If fertilized under these conditions, it would give rise to ovarian preg- 

 nancy. These three abnormal conditions are spoken of as cases of extra- 

 uterine pregnancy. 



The periods of ovulation, or extrusion of the ovum from the Graafian 

 follicle and ovary, which occur at more or less regular successive intervals, 

 are attended by certain changes in the mucous membrane of the cavity of the 

 body of the uterus. These changes constitute menstruation. 



Maturation of the Ovum. 



The ovum, as it leaves a Graafian follicle, represents a primary 

 oocyte. In order to become mature and capable of being fertilized 

 it has to undergo certain changes which affect primarily the ger- 

 minal vesicle or nucleus, these changes being of the nature of 

 karyokinetic or mitotic cell-division. Briefly stated, the changes 

 consist in the ovum undergoing two unequal cell-divisions. In 

 other words, the ovum extrudes two polar bodies, one after the 

 other, these being minute spherical globules, which are abortive ova. 



When the first cell - division 

 Primaryoocytc ^^^^^ pj^^^g ^^iQ primary oocyto 



(ovum) divides into two cells of 

 unequal size. The large cell is 

 the secondary oocyte, and the 

 small one is the first polar body. 

 At the second cell-division the 

 secondary oocyte likewise di- 

 vides into two cells, of unequal 

 size. The large cell is the 

 mature ovum, or female pro- 

 nucleus, and the small one is 

 the second polar body. The 

 first division may take place 

 whilst the ovum is in a Graafian 

 follicle, or, like the second, in 

 the Fallopian tube. 



The polar bodies are ex- 

 truded from the ovum, and, 

 prior to the extrusion of the 

 second, the first polar body 

 usually undergoes an equal division, so that utimately there are 

 three polar bodies. These, being abortive ova, so^n disappear. 



The final result of the foregoing two divisions is the formation of 

 four cells. One of these, of large size, is the mature ovum, or female 

 pronucleus, which is capable of activity, and the other three, of 

 small size, are inactive polar bodies, or abortive ova, the first polar 

 body having, as a rule, divided into two. Prior to the first cell- 

 division, or extrusion of the first polar body, the germinal vesicle 

 or nucleus moves towards the periphery of the primaty oocyte. 



Mature Ovum 



Derivatives of ist 

 Polar Body 



Fig. 663. — Maturation of the Ovum. 



