262 The Vertebrate Organ Systems 



The Egg. — The individual human egg is a small sphere about 0.2 

 mm. in diameter. Surrounding the egg as it is released from the ovary 

 are several small follicular cells. Immediately next to the egg is a heavy 

 membrane, the sona pellucida. The cytoplasm has some yolk granules, 

 and the nucleus is approximately in the center of the cell. 



Ova are first released from the ovary at puberty ; this release is ac- 

 complished by the first appearance of the menstrual cycle and the de- 

 velopment of the familiar secondary sexual characters. The ova are 

 released at approximately twenty-eight-day intervals (except during 

 pregnancy) until the menopause, which occurs normally between the 

 forty-fifth and fiftieth years. At this time, the follicles of the ovary dis- 

 integrate, menstrual cycles stop, and the various sex hormones are grad- 

 ually withdrawn. 



The Mammary Glands. — Although the mammary glands are not 

 directly associated with the reproductive organs, they function as acces- 

 sory organs after the birth of young among the mammals. At birth they 

 are very small and they enlarge at puberty. During pregnancy, further 

 enlargement occurs, and the glands become functional after parturition. 

 Their functioning is dependent upon the release of the hormone prolactin 

 by the anterior pituitary. 



Menstruation. — During the reproductive period of the human 

 female, the mucosal lining of the uterus undergoes definite cyclical 

 changes related to ovulation. These cycles take approximately twenty- 

 eight days for completion. Each cycle begins with the start of men- 

 struation which lasts for four to five days. At this time, the endo- 

 metrium of the uterus undergoes extensive degeneration and is sloughed 

 off. A new follicle begins to develop in the ovary, and the corpus luteuni 

 of the previously expelled ovum gradually disappears. After men- 

 struation ceases, there is a resting period during which the endometrium 

 begins to be restored and the new ovum matures. This period lasts 

 from the fourth to eighth day of the cycle. The next period, the interval, 

 lasts until about the eighteenth day. At this time, the endometrium 

 regains its original thickness. Ovulation occurs on about the four- 

 teenth day of the cycle. The last period, taking place from the eight- 

 eenth to the twenty-eighth day is known as the premenstrual period. At 

 this time, the corpus luteuni is forming, the released ovum is disinte- 

 grating, and the endometrium gradually starts breaking down. 



As was noted previously, these changes are under the control of 

 the endocrines. 



