CORPUS LUTEUM 331 



right angles to the circumference of the egg, and this layer is the 

 corona radiata already mentioned. The zona pellucida is just 

 underneath the corona. 



Usually a Graafian follicle contains only one ovum. The 

 follicles and their contained ova begin to be formed early in 

 fetal life. Probably none are newly formed after the child is two 

 years old, but they are undeveloped before puberty. It is esti- 

 mated that some 72,000 follicles and ova exist in the two ovaries 

 of the average woman; but of these not more than 400 reach 

 full development, the others undergoing retrograde changes and 

 disappearing. 



Up to puberty the follicles and ova are small, but at that time 

 some of them begin to enlarge, and at more or less regular inter- 

 vals one of these follicles bursts and allows the escape of its con- 

 tained ovum into the fimbriated extremity of the Fallopian tube 

 a process known at ovulation. Previous to its rupture the 

 Graafian follicle has been enlarging. It is always located in the 

 cortical part of the ovary, but it may now not only form a dis- 

 tinct protrusion above the surface of the organ, but may by its 

 size encroach upon the medullary portion. It may at this time 

 have a diameter of half an inch. Meantime the more superficial 

 part of the tunica vasculosa has been undergoing fatty degenera- 

 tion, has lost its blood supply and become very thin. Here 

 rupture occurs, and the mature ovum, ready for impregnation, 

 escapes upon the surface of the ovary. 



Corpus Luteum. When the ovum has been extruded hem- 

 orrhage occurs, filling the empty follicle with blood. By con- 

 traction of the extra-vesicular adjacent tissue the walls of the 

 Graafian follicle become folded into the cavity. Soon prolifera- 

 tion of the cells of the follicular wall takes place into the blood 

 clot, vascular loops are formed, and the tunica vasculosa itself 

 becomes greatly hypertrophied. The clot later disappears and 

 the mass then has a yellowish color and is known as the corpus 

 luteum. 



