GROWTH AND REPRODUCTION 771 



contractions, by this means passing down the mucous content of the 

 uterus to the os uteri. The uterus is also said to raise itself, and 

 sink more deeply into the vagina, possibly to facilitate the entrance 

 of the semen into it. Finally, a copious secretion takes place from 

 the uterus and walls of the vagina about the same time as the ejacula- 

 tions of the semen by the male. The sexual act is usually attended 

 with considerable nervous excitement, and followed by a period of 

 lassitude . 



Fertilization. Fertilization consists in the union of the male 

 and female gametes. In the higher animals, the spermatozoa of the 

 male are deposited by the sexual act in the neighbourhood of the 

 uterine cervix. Stimulated probably by the vaginal and uterine 

 secretions under chemiotactic influence, they seek out the ovum. 

 They ascend the uterus " against the stream." The activity of the 

 cilia of the uterine mucous membrane is such as to impede their 



FIG. 477. MATURE OVUM OF A BAT, SHOWING THE SEPARATED POLAR BODIES, THE 

 FEMALE PRONUCLEUS AND THE FORMATION OF THE MALE PRONUCLEUS FROM. 

 THE HEAD OF THE SPERMATOZOON. (Keith, after Van der Stricht.) 



In this case the tail-piece has not been left behind. 



progress. Only one spermatozoon is necessary for fertilization. The 

 race is to the strong, and perhaps to the fleet, since it is calculated 

 that they move from 16 to 20 centimetres in an hour about the 

 distance from the os uteri to the Fallopian tube. It may be, however, 

 that the winning spermatozoon is deposited within the uterus itself 

 as the result of the relaxation of the cervix uteri which takes place 

 during coitus. This, however, is not necessary, since fertilization 

 may take place when the spermatozoa are deposited only in the 

 entrance to the vagina. 



The time taken for fertilization is not known, but it is known 

 that the spermatozoa may remain active in the vagina for a period 

 of three weeks. The actual fertilization is said to take place most 

 commonly in the Fallopian tube, and not within the uterine cavity. 

 The head and middle piece of the fertilizing spermatozoon pass into 



