• Fertilisation is preceded by copulation in which the erect penis is inserted into the vagina.
• This leads to ejaculation of semen.
• The sperms swim through the female's genital tract to the upper part of the oviduct.
• The head of the sperm penetrates the egg after the acrosome releases lytic enzymes t dissolve the egg membrane.
• The tail is left behind.
• Sperm nucleus fuses with that of the ovum and a zygote is formed.
• A fertilisation membrane forms around the zygote which prevents other sperms from penetrating the zygote.


Implantation:
• After fertilisation the zygote begins to divide mitotically as it moves towards the uterus.
• It becomes embedded in the wall of the uterus a process called implantation.
• By this time the zygote is a hollow ball of cells called blastocyst or embryo.
• In the uterus the embryo develops villi which project into uterus for nourishment later the villi and endometrium develop into placenta.


Embryonic membranes
• Embryonic membranes develop around the embryo.
• The outermost membrane is the chorion which forms the finger-like projections (chorionic villi) which supply nutrients to the embryo.
• The amnion surrounds the embryo forming a fluid filled cavity within which the embryo lies.
• Amniotic cavity is filled with amniotic fluid.
• This fluid acts as a shock absorber and protects the foetus against mechanical injury.
• It also regutates temperature.
• The chorionic villi, allantois together with the endometrium from the placenta.
• The embryo is attached to the placenta by a tube called umbilical cord which has umbilical vein and artery.
• The maternal blood in the placenta flows in the spaces lacuna and surrounds capillaries from umbilical vein and artery.
• The umbilical cord increase in length as the embryo develops.

Role of placenta
Protection
• Maternal blood and foetal blood do not mix.
• This ensures that the pathogens and toxins from maternal blood do not reach the foetus.
• The placenta allows maternal antibodies to pass into the foetus, providing the foetus with immunity.
Nutrition
• The placenta facilitates the transfer of nutrients from maternal blood to foetus.
Excretion
• Placenta facilitates the removal of nitrogenous wastes from the foetus' blood to maternal blood.
Gaseous exchange
• Oxygen from the maternal blood diffuses into the foetal blood while carbon (IV) oxide from foetal blood diffuses into maternal blood.
Production of hormones
• Placenta produces progesterone and oestrogen.

Gestation period
• The period between conception and birth is called gestation.
• In humans gestation takes nine months (40 weeks).
• The embryo differentiates into tissues and organs during this period.
Week 1 to 3:
• Zygote divides to form blastocyst.
• Implantation takes place.
• The three germ layers form endoderm, mesoderm and ectoderm.
• Nervous system starts to form.
Week 4 to 7:
• Development of circulating and digestive systems.
• Further development of nervous system, formation of sensory organs,
• All major internal organs are developed.
• At week 5, heartbeat starts.
Week 8 to 24:
• All organs well developed including sex organs.
• Hair, finger and toe nails grow.
• Foetus move and eyelids open.
Week 25- 30:
• The fully developed foetus responds to touch and noises and moves vigorously.
• The head turns and faces downwards ready for birth.
Week 31-40:
• Foetus increases in size.
• Birth occurs.

The events that take place to facilitate parturition
• Near birth the placenta produces less progesterone
• Oxytocin hormone is produced by posterior lobe of pituitary gland
because progesterone level has decreased, the uterus becomes sensitive to oxytocin
• oxytocin causes the contraction of the uterus (myometrium) these contractions are called labour pains,
just before parturition the head turns downwards
• The contractions eventually push the baby through the vagina
• Amnion breaks and amniotic fluid is released.
• Oxytocin dilates the cervix
• Foetus is expelled through cervix with head coming out first
• Finally the whole infant comes out
• The umbilical cord is cut and the placenta is expelled as afterbirth