During the development of an embryo,which of the following occurs first?

Which feature distinguishes a morula from a blastula?

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:

What does the corpus luteum secrete?

Identify $X$ from the figure:

The type of placenta in humans is: