Explain the difference between the following terms:
$(1)$ Menstrual flow - Menstrual cycle
$(2)$ Embryo - Foetus

(N/A) $(1)$ Menstrual flow: The menstrual flow results from the breakdown of the endometrial lining of the uterus and its blood vessels,which forms a liquid that is discharged through the vagina. Menstruation occurs only if the released ovum is not fertilized.
- Menstrual cycle: The cyclic changes occurring in the inner wall (endometrium) of the uterus,driven by fluctuations in the levels of female sex hormones (estrogen and progesterone),are called the menstrual cycle.
$(2)$ Embryo: The fertilized ovum during the first $8$ weeks of development is called an embryo.
- Foetus: The stage of the developing organism after $8$ weeks of gestation is called a foetus.