662 THE HUMAN BODY. 



remaining to the germ plasma brought by the spermatozoon 

 may be supposed to differ in different instances and account 

 for individual differences in the offspring: thus some physical 

 basis for the facts of variation as well as of heredity would 

 be obtained. 



Impregnation. The fertilized ovum continues its descent 

 to the uterine cavity, but, instead of lying dormant like the 

 unfertilized, segments (p. 29), and forms a morula. This, 

 entering the womb, becomes imbedded in the soft, vascular 

 mucous membrane from which it imbibes nourishment, and 

 which, instead of being cast off in subsequent menstrual dis- 

 charges, is retained and grows during the whole of pregnancy, 

 having important duties to discharge in connection with the 

 nutrition of the embryo. 



Sexual congress is most apt to be followed by pregnancy if 

 it occur immediately after a menstrual period; at those times 

 a ripe ovum is usually in the Fallopian tube, near the upper 

 end of which it is probably fertilized in the majority of cases. 

 There is some difference of opinion as to whether the rupture 

 of the Graafian follicle occurs most frequently immediately 

 before the appearance of the menstrual flow, or towards its 

 close ; but the preponderance of evidence favors the latter view. 

 The menstrual process probably is a special preparation of the 

 womb for the reception of an embryo and its nourishment. 

 There is, however,, evidence that ova are ocfisionally discharged 

 at other than the Tegular monthly periods of ovulation and 

 may be fertilized and cause a pregnancy. 



Pregnancy. When the mulberry mass reaches the uterine 

 cavity the mucous membrane lining the latter grows rapidly 

 and forms a new, thick, very vascular lining to the womb, 

 known as the decidua. At one point on this the morula be- 

 comes attached, the decidua growing up around it. As preg- 

 nancy advances and the embryo grows, it bulges out into the 

 uterine cavity and pushes before it that part of the decidua 

 which has grown over it (the decidua reflexa) ; at about the 

 end of the third month this coalesces with the decidua lining 

 the opposite sides of the uterine cavity so~that the two can no 

 longer be separated. That part of the decidua (decidua 

 serotina) against which the morula is first attached subsequently 

 undergoes a great development in connection with the forma- 

 tion of the placenta (see below). Meanwhile the whole uterus 

 enlarges; its muscular coat especially thickens. At first the 



