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

Which of the following structures forms the birth canal after fertilization?

The corpus luteum secretes ........

The given figure shows the human foetus within the uterus with few structures marked as $A, B, C$ and $D$. Which of the following options shows the correct labeling?

Assertion: Corpus luteum degenerates in the absence of fertilization.
Reason: Progesterone level decreases.

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 . . . . . . .