REPRODUCTION 457 



pubis and in front of the anus. It has spongy, vascular walls and 

 is invested by a loose sheath of skin, the foreskin or prepuce, which 

 hangs forwards over the tip or gland of the organ. In sexual 

 excitement the penis is stiffened or erected by the flow of blood 

 into venous spaces which it contains, and in this state is thrust 

 rhythmically in and out in the vagina of the female. This causes a 

 reflex contraction of muscles of the vasa deferentia, which ejacu- 

 lates the semen containing the sperm into the female. The whole 

 act is known as coition or copulation. The ovaries are small, oval 

 bodies attached behind the kidneys to the dorsal abdominal wall, 

 and show on their surface little blister-like projections, known as 

 Graafian follicles, each of which contains a microscopic ovum. 

 The oviducts open into the abdominal cavity by wide, funnel- 

 shaped fimbriated openings just outside the ovaries. When the 

 ova are ripe the follicles burst and discharge the ova into the 

 funnels, which at that time extend over them. The first section 

 of each duct is narrow and gently sinuous and is known as the 

 Fallopian tube. It runs backwards and enlarges into the uterus, a 

 vascular- walled structure which joins its fellow in the middle line 

 anteriorly to the bladder to form the vagina. This passes back- 

 wards within the pelvic girdle above the neck of the bladder, with 

 which it presently unites to form a short urinogenital canal or 

 vestibule, which opens at the vulva. On its ventral wall lies the 

 small, rod-like clitoris and on the dorsal wall two small Cowper's 

 glands. 



The female mammal will normally receive the male only at 

 certain periods, when she is said to be in oestrus or on heat, and 

 in some mammals, such as the sheep, the ripening of ova and 

 their discharge, or ovulation, takes place automatically at these 

 times ; the rabbit will copulate at any time, and ova are not 

 discharged except after coition. The spermatozoa travel up the 

 oviducts and fertilisation takes place at the upper ends of the 

 latter. The ova then pass down the oviducts, in which they 

 segment. At the end of the third day they reach the uterus. 

 Here at first they He free. On the eighth day, however, they 

 begin to become attached to the uterine wall, and in the course 

 of the next few days there is formed in connection with each 

 of them a special organ, known as the placenta, in which blood 

 vessels derived from the mother and the developing young 

 are in very close and extensive contact. Through the thin walls 

 of the two sets of blood vessels interchange of fluid and gaseous 



