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Human embryonic stem cell colonies in different stages of development are shown in 1998. The maintenance of embryonic stem cells in their undifferentiated form was reported in the November 6, 1998 Science by a team from the University of Wisconsin-Madison. Here, embryonic stem cell colonies sometimes include a core of undifferentiated cells surrounded by a margin of differentiated cells, such as the small colony at right in Figure B. (KEYSTONE/AP Photo/UW-Madison University Communications) === ===