In indeterminate cleavage,the fate of blastomeres is fixed at which of the following stages?

When is the heart formed in a human embryo?

The blood flowing in the umbilical cord is:

The first sign of the growing foetus may be noticed by listening to the heart sound carefully through the stethoscope. The embryo's heart is formed . . . . . . .

In which stage of development do the embryonic cells form the germinal layers?

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: