How many cleavages are required to form a $16$-celled blastomere?

$A$: Placenta is connected to the embryo through an umbilical cord which helps in the transport of substances to and from the embryo.
$R$: Placenta acts as an endocrine tissue.

Which one of the following statements with regard to embryonic development in humans is correct?

The brain and spinal cord are derived from which germ layer?

The villi which are formed by the allantochorion to form an intimate connection with the uterine endometrium function as:

Which of the following is a source of the hormone progesterone?