blastema

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A mass of undifferentiated cells from which an organ or a body part develops, either in normal development or in the regeneration of a lost body part.
• noun A structureless substance from which it was formerly believed that cells are formed.

from The Century Dictionary.

• noun In bot.: Originally, the axis of an embryo, consisting of the radicle and the growing-point at its summit.
• noun In later use, the initial point of growth from which any organ or part of an organ is developed.
• noun Sometimes, the thallus of cryptogamous plants.
• noun In anatomy and physical, the bioplasm or protoplasm of a germinating ovum; the substance of the blastomeres, blastoderm, etc.; granular formative material.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Biol.) The structureless, protoplasmic tissue of the embryo; the primitive basis of an organ yet unformed, from which it grows.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physiology A clump of undifferentiated cells or blasts, from which an organ or body part will develop, either during the normal growth of an embryo or in the regeneration of a lost body part.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a mass of undifferentiated cells from which an organ or body part develops