In vertebrates the nails are formed from

An avian blastula is called

The embryo of man is protected by

Test tube baby means a baby born when

The only human system that is derived from all the three germ layers is

$A$ : If both the ovaries are removed after the first trimester of pregnancy, still there would be normal growth of foetus.
$R$ : After first trimester placenta secretes sufficient progesterone