An embryonic stem cell which may be induced to differentiate homogeneously into a desired primary cell line. The embryonic stem cell may be engineered with DNA, which encodes a protein or polypeptide which promotes differentiation of the stem cell into a specific cell line, such as, for example, a neuronal cell line, a muscle cell line, or a hematopoietic cell line. The DNA may encode a transcription factor found in the particular cell line. In another alternative, a desired cell line is produced from embryonic stem cells by culturing embryonic stem cells under conditions which provide for a three-dimensional network of embryonic stem cells, and then stimulating embryonic stem cells with an agent, such as retinoic acid, or dimethylsulfoxide, which promotes differentiation of the embryonic stem cells into the desired cell line, such as, for example, a neuronal cell line, or a muscle cell line.