336 REPRODUCTION 



than seven days unless impregnated, and if impregnation is to 

 occur, it must take place within the first week after ovulation. 

 Since, therefore, ovulation and menstruation usually occur to- 

 gether, and since impregnation probably occurs about the be- 

 ginning of menstruation, 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 evident that this calculation at best gives only the approxi- 

 mate 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 prolonged 

 that the time of its deposit with reference to menstruation very 

 probably has little to do with whether or not conception shall 

 occur. 



Impregnation. The term impregnation, or fertilization, 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 Fal- 

 lopian tube, and almost always in the outer third. The male 

 element, the spermatozoon, seeks and penetrates the female ele- 

 ment, the ovum. It is the blending of the nuclei (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 usually to enter the ovum. At is ap- 

 proaches the vitelline membrane, head first, the protoplasm of the 

 ovum swells up into a prominence to meet it. The fertilizing 

 spermatozon 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 the male element, and they coalesce to become the segmen- 

 tation nucleus. Impregnation has now taken place. The seg- 

 mentation nucleus represents a new being. It contains anatom- 



