The placenta helps in the maintenance of pregnancy by secreting a hormone,known as

All extra-embryonic membranes:

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

Identify the structure that connects the embryo and the uterine wall in mammals.

After one month of pregnancy,the embryo's $A$ is formed. By the end of the $B$ month of pregnancy,the foetus develops limbs and digits. By the end of $C$,most of the major organ systems are formed,for example,the limbs and external genital organs are well-developed. By the end of $D$,the body is covered with fine hair,eyelids separate,and eyelashes are formed. Here $A$ and $D$ refer to:

By the end of the second month of pregnancy,the foetus develops........