FERTILISATION 359 



copulation. It is probable that in women the period 

 immediately following menstruation, corresponding to 

 oestrus in lower animals, is the only period during which 

 fertihsation can occur. The descent of the ovum down the 

 Fallopian tube coincides with the ascent of the spermatozoa. 



Fertilisation 



During coitus the spermatozoa deposited in the vagina 

 are sucked into the uterus by peristaltic contraction of 

 this organ initiated reflexly by contact with the male, 

 the efferent path being the sympathetic. They travel up 

 into the Fallopian tubes, overcoming by the propulsive 

 action of their tails the downward current produced by 

 the ciha of the female passages. In the tube they meet 

 the matured ovum on its way down from the ovary. 



Of the many million spermatozoa which enter the 

 female organs only one enters the ovum. 



After impregnation, the tail is absorbed and the head, 

 now known as the male 'pro-mwleus, fuses with the female 

 pro-nucleus. In this process the number of chromosomes, 

 which in each element has been reduced by a half during 

 maturation, is restored to the number characteristic of 

 the species. The nucleus thus formed is called the seg- 

 mentation-nucleus. From the fertihsed cell or oosperm 

 arises the new generation. 



CHANGES IN THE PREGNANT UTERUS 



As the fertihsed ovum passes down the Fallopian tubes 

 changes occur in the mucous membrane of the uterus, 

 preparatory to the embedding of the ovum within it. 

 The stroma becomes transformed into a mass of decidual 

 cells — large cells with small nuclei. The glands enlarge, 

 the epithehum prohferates, and the blood-vessels are 

 dilated. In this way the mucous membrane becomes 

 greatly thickened. 



By the time it reaches the uterus the ovum has developed 



