Provide definitions/explanations for the following terms:
$(1)$ Menstrual cycle
$(2)$ Foetus

(N/A) $(1)$ The menstrual cycle refers to the cyclic changes that occur in the endometrium (the inner wall of the uterus) due to fluctuations in the levels of the female sex hormones,estrogen and progesterone.
$(2)$ The developing human organism from the end of the eighth week of gestation until birth is referred to as a foetus.