654 TEXT-BOOK OF PHYSIOLOGY. 



of the chromatin material. The extruded portions are known as the 

 polar bodies. The non-extruded portion of the chromatin material 

 is known as the female pronucleus. The succession of changes which 

 the nucleus undergoes is termed maturation. As the nucleus is 

 regarded as the part of the ovum which transmits parental character- 

 istics it is assumed that the extrusion of a portion of the nuclear 

 material is a means by which an excess of inherited substance is 

 prevented. 



Menstruation. Menstruation is a periodic discharge of blood 

 and mucus from the surface of the mucous membrane of the uterus, 

 and occurs about every twenty-eight days. The duration of the 

 menstrual period extends over four or five days and the amount of 

 blood discharged varies from 180 c.c. to 200 c.c. Menstruation is 

 usually an accompaniment of ovulation, though the latter process may 

 take place independently of the former. It is characterized by both 

 local and systemic changes. The local changes are most marked 

 in the uterus, the mucous membrane of which increases in thickness 

 from a proliferation of the connective tissue and a hyperemic condi- 

 tion of the blood-vessels. Subsequently to these changes the epithe- 

 lial surface, as well as the more superficial portions of the connective 

 tissue, undergo degeneration and exfoliation, after which the finer 

 blood-vessels rupture and permit of an escape of blood into the 

 uterine cavity. At the end of the menstrual period regenerative 

 changes set in which continue until the normal condition of the 

 mucous membrane is reestablished. 



The Corpus Luteum. With the rupture of the Graafian follicle 

 there is an effusion of blood into the follicular cavity which soon 

 coagulates, loses its color and assumes the characteristics of fibrin. 

 The walls of the follicle, which have become thickened from the 

 deposition of a reddish-yellow glutinous substance, now become con- 

 voluted and undergo a still further hypertrophy, until they encroach 

 upon and almost obliterate the follicular cavity. In a few weeks the 

 mass loses its red color and becomes decidedly yellow, when it is 

 known as the corpus luteum. With the continuance of reparative 

 changes this body gradually disappears until at the end of two months 

 nothing remains but a small cicatrix on the surface of the ovary. 

 Such are the changes in the follicle if the ovum has not been im- 

 pregnated. 



The corpus luteum, after impregnation has taken place, undergoes 

 a much slower development, becomes larger, and continues during 

 the entire period of gestation. The difference between the corpus 

 luteum of the unimpregnated and pregnant condition is expressed in 

 the following table by Dalton : 



