• Early development of a human zygote:

Fertilisation:

1. Definition: The union of sperm nucleus and ovum nucleus to form a zygote
2. Fertilisation occurs at the Fallopian tube
3. Ovum is surrounded by millions of sperms, but ony one will successfully penetrate the ovum wall
4. When the penetration is successful, a barrier known as the fertilisation membrane is formed to prevent the penetration of other sperms

• Early embryo development: 

1. 36 hours after fertilisation: the zygote undergoes repeated mitosis as it travels along the Fallopian tube towards the uterus
2. The zygote continuosly undergoes mitosis forming embryo with two cells,four cells and eight cells and resulting in the formation of a solid mass known as morula
3. On the 5th day after fertilisation, the morula develops into a ball of 100 cells with a cluster of inner cells known as blatocyst. Only one part of blatocyst will develop into foetus
4. Implantation: The blatocyst is implanted onto the endometrium, 7 days after fertilisation. The inner cell masss develops into an embryo. Embryo will absorb the nutrients directly form the endometrium tissue
5. 8 weeks after fertilisation, a foetus is formed and obtains nutrients through placenta
6. At 38 weeks after fertilisation, the foetus is well developed into full term, waiting for birth to take place

Role of human chorionic gonadotrotin (HCG) during pregnancy process:

1. If fertilisation is occured, zygote formed divides into embryo

• The embryo begins to produce human chorionic gonadotrotin (HCG) hormone for maintaining the function of corpus luteum
• The corpus luteum continue to function by secreting progesterone and oestrogen in the first two months of pregnancy
• The function of corpus luteum will be taken over by placenta at the 4th month of pregnancy

Functions of placenta and umbilical cord in the development of foetus:

1. During foetus development, blatocyst forms chorionic villi into endometrium to obtain nutrients and oxygen from maternal blood
2. Chorionic villi will form placenta in the 4th week of pregnancy
3. Placenta is the site of substances exchange between foetal blood and maternal blood
4. Useful substances such as nutrients,oxygen and antibodies are transported from maternal blood to foetal blood through placenta and umbilical cord
5. While, waste substances such as urea and carbon dioxide are transported from foetal blood to maternal blood through placenta and umbilical cord
6. Umbilical cord has umbilical arteries and umbilical vein

Necessity of having seperate foetal and maternal circulatory system:

1. Protects foetus's fine blood vessels from being damaged by high maternal blood pressure 
2. Prevents agglutination if blood group of foetus is incompatible to maternal blood group
3. Filters certain pathogens and harmful substances from entering foetal blood