Cucumber; differentiating the leaves

There were three cucumber seedlings to start with, now left with two. The third decided to wilt and die a week ago. I am getting at how leaves take its true form at the second pair, while the baby leaves, the one emerging from the cotyledon remains cute, tough and thick.

Cucumber seems to be cousin of bitter gourd, judging by the leaves, and their very familiar yellow flower.

bangchik