466 THE HUMAN EMBRYO. 



Concerning the spermatozoa, we have very little precise 

 knowledge. It is known that spermatozoa, introduced into the 

 vagina, may retain their vitality, and presumably their fertilising 

 power as well, for a week ; and the fact that successful impreg- 

 nation may occur at any time in the menstrual cycle, strength- 

 ened by the analogy of other animals, suggests that human 

 spermatozoa may retain their power for considerably longer 

 periods. It is stated that ripe spermatozoa may remain for 

 months in the testis before being discharged, without losing 

 their fertilising power. 



The time taken by the spermatozoa to travel along the 

 vagina, uterus, and Fallopian tube to the ovary, is not known, 

 but is probably very short ; in the rabbit it does not occupy 

 more than a quarter of an hour to two hours. 



If the ovum is not fertilised it soon dies. How long an 

 ovum may retain its vitality, and capacity for fertilisation, is not 

 known; indeed, no unfertilised human ovum has yet been seen, 

 outside the ovary. Some experiments of Bischoff, on lower 

 Mammals, point to the conclusion that, in these, the ovum, if not 

 fertilised, dies in the lower part of the Fallopian tube, before 

 reaching the uterus. Assuming that the human ovum also dies 

 shortly before reaching the uterus ; and assuming further, as is 

 done by most authorities, that the human ovum takes at least 

 eight days to travel down the Fallopian tube, it may be stated 

 that the human ovum probably retains its vitality, and power 

 of being fertilised, for some time, perhaps a week, after discharge 

 from the ovary ; but ultimately loses it, probably before reaching 

 the uterus. This is, however, at present little more than specu- 

 lation. 



If the above considerations prove well founded, and if, as 

 suggested above, the length of time during which an ovum 

 remains alive and fertilisable, after leaving the ovary, is less 

 than the interval between two successive periods of ovulation, it 

 will follow that there must be certain times during which there 

 are ova ready to be fertilised, and certain times during which 

 there are none ; i.e. that fertilisation can only be effected at 

 certain recurring periods, and cannot occur in the intervals 

 between these periods. 



Concerning the respective lengths of these periods we have 



