IMPREGNATION 337 



for ten lunar months and dates from the time of impreg- 

 nation (conception), but that time cannot in any case be 

 fixed upon with precision. The vitality of the ovum is 

 thought not to last longer than seven days unless impreg- 

 nated, and if impregnation is to occur, it must take place 

 within the first week after ovulation. Since, therefore, ovu- 

 lation and menstruation usually occur together, and since im- 

 pregnation probably occurs about the beginning of menstru- 

 ation, we reckon from the first day of the last menstruation 

 280 days forward to determine the probable time of labor. 

 This is equivalent to adding nine calendar months and seven 

 days to the first day of the last menstrual period. It is evi- 

 dent that this calculation at best gives only the approximate 

 time. 



While fertilization probably occurs at the time mentioned, 

 the spermatozoon effecting fecundation may have been in the 

 female genital tract for weeks. Its vitality here is so pro- 

 longed that the time of its deposit with reference to men- 

 struation very probably has little to do with whether or not 

 conception shall occur. 



Impregnation. The term impregnation, or fertilisation, 

 or fecundation, is used to signify that union of the male and 

 female sexual cells which makes possible the development 

 of a new human being. Normally impregnation takes place 

 in the Fallopian tube, and almost always in the outer third. 

 The male element, the spermatozoon, seeks and penetrates 

 the female element, the ovum. It is the blending of the nu- 

 clei (pronuclei) which is essential. Spermatozoa in large 

 numbers swarm around the ovum and several at least enter 

 the perivitelline space. Only one, however, is destined usu- 

 ally to enter the ovum. As it approaches the vitelline mem- 

 brane, head first, the protoplasm of the ovum swells up into 

 a prominence to meet it. The fertilizing spermatozoon makes 

 its way through the vitelline membrane, losing its tail in the 

 passage, and becomes the male pronucleus. The female 

 pronucleus now advances from its central position to meet 



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