From which part of the embryvo are embryonic stem cell lines derived?

Undifferentiated inner cell mass

Explanation:

Human embryo has two types of cells in its initial stage, trophoblast and inner cell mass.  Trophoblast forms the placenta and inner cell mass form the entire human body. Inner cell mass are pluripotent in nature i.e. they can give rise to any body part under right conditions.

Embryonic stem cells are derived from the inner cell mass. They are derived from pre implantation embryo since after implantation the inner cell mass begins to differentiate. Embryonic stem cells can replicate indefinitely without differentiating. They can give rise to all the three germ layers: ectoderm, mesoderm and endoderm. They can treat a number of disorders like genetic disorders, Parkinson's, spinal cord injury, cancers etc.

your answer is "the 1950's"