COORDINATION 



441 



Since we are dealing with a cycle, de- 

 scription can start at any point in it (Fig. 

 16-32). The termination of the menstrual 

 flow may arbitrarily be taken as the start- 

 ing point for this discussion. At this point 

 a new Graafian follicle begins to form in 

 the ovary and as it grows in size it produces 

 more and more estrogen. Maturity is 

 reached in about 12 to 18 days at which 

 time a rupture occurs in its wall and the 

 egg is released. This marks the end of estro- 

 gen production from this structure but not 

 of the hormone itself. The empty follicle is 

 quickly converted into the corpus luteum, 

 which continues to produce in increasing 

 amounts the closely related hormone, pro- 

 gesterone, from the 15th to the 26th day. 

 These hormones, in addition to bringing on 

 the changes already referred to at puberty, 

 are thus also responsible for the rhythmic 

 menstrual cycle. The production of both 

 estrogen and progesterone have a stimulat- 

 ing effect on the walls of the uterus, causing 

 it to proliferate and to become highly 

 vascular in preparation for the fertilized 

 egg, if and when it makes its way into the 

 uterine cavity. What happens from this 

 point forward depends on whether or not 

 the egg is fertilized. 



If the egg is not fertilized the corpus 

 luteum suddenly retrogresses and proges- 

 terone is reduced to zero during the 25th- 

 27th days of the cycle. This results in the 

 sloughing off of the uterine wall known as 

 the menstrual flow, which continues over a 

 period of 4-5 days. Another Graafian follicle 

 then begins to grow and the cycle is started 

 over again. All this is complicated by the 

 pituitary hormones which will be discussed 

 a little later. It might seem that a rather 

 elaborate preparation is made each month 

 for the event of pregnancy and that an un- 

 necessary waste results when fertilization 

 fails to occur. One speculates why some- 

 thing a little less pretentious might not be 

 satisfactory until it became certain that the 

 great climax, fertilization, had taken place. 

 But, even though there seems to be little 



justification in its complicated machinery, 

 this is the way it is set up. 



If the egg is fertilized as it passes down 

 the oviduct, the corpus luteum is retained 

 and goes right on producing progesterone 

 until just a few days before the end of 

 gestation. The zygote is implanted in the 

 uterine wall wherever it happens to touch. 

 In fact, the highly vascular wall is so recep- 

 tive to tiny particles that almost any small 

 object is readily picked up by it at this 

 time. The walls also produce mucus rich 

 in glycogen, which probably acts as a 

 source of energy for the early stages of the 

 embryo until it gains a secure foothold and 

 can withdraw nourishment through its pla- 

 centa. With the developing stages of preg- 

 nancy, progesterone continues to cause fur- 

 ther accommodations of the uterine wall for 

 the enlarging embryo. It also causes the 

 mammary glands to increase in size, pre- 

 vents any further Graafian follicles from 

 forming, and inhibits uterine contractions. 

 As the end of pregnancy approaches, the 

 corpus luteum ceases to produce any more 

 progesterone and the uterine wall, which 

 has during this time become an accessory 

 in producing the hormone, reduces its out- 

 put. This precipitates changes which are 

 similar to the beginning of menstruation, 

 that is, in the absence of progesterone the 

 uterine wall begins to degenerate, therefore 

 becoming incapable of nourishing the fetus 

 any longer. Furthermore, without the in- 

 hibiting effect of progesterone, the muscles 

 of the uterine wall begin powerful contrac- 

 tions which eventually result in expelling 

 the fetus. The production of milk by the 

 mammary glands occurs after birth due to 

 another hormone, lactogen, which is pro- 

 duced by the pituitary gland and will also 

 be discussed later. 



It takes some time after the birth of the 

 offspring for the hormones to readjust them- 

 selves and the menstrual cycle once again 

 to reestablish itself. This usually does not 

 occur until the amount of lactogen from the 

 pituitary subsides, which means, of course, 



