18 THE HUMAN BODY. 



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Jexual congress is most apt to be followed by pregnancy 

 if it occur immediately before or after a menstrual period; 

 at those times a ripe ovum will likely be in the Fallopian 

 tube, near the upper end of which it is probably fertilized 

 in the majority of casesA 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 pre- 

 ponderance of evidence favors the latter view; if this be so 

 coition will occur under the most favorable conditions for 

 conception if it take place on the day following the cessation 

 of a menstrual period. There is, however, evidence that 

 ova are occasionally discharged at other than the regular 

 monthly periods of ovulation. There is, therefore, no time 

 during a woman's life from puberty to the climacteric, 

 except pregnancy or lactation, when sexual congress may 

 not result in impregnation 



The exact parts taken by the male and female elements 

 respectively in the formation of the embryo are not known 

 with certainty; seeing that the ovum is much larger than 

 the spermatozoon it is not uncommon to speak of the latter 

 as a mere stimulant or excitant, arousing the egg-cell to 

 developmental activity; but the definite characteristics often 

 inherited by children from the father show that the sperma- 

 tozoon is much more than that; materials derived from it 

 are no doubt an essential constituent of the compound mass 

 which develops into the new human being. 



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 

 becomes attached, the decidua growing up around it. As 

 pregnancy 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 meets the decidua 

 lining the opposite sides of the uterine cavity and the two 

 grow together. That part of the decidua (decidua serotina) 

 Against which the morula is first attached subsequently un- 



