mesoderm

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

• noun The middle embryonic germ layer, lying between the ectoderm and the endoderm, from which connective tissue, muscle, bone, and the urogenital and circulatory systems develop.

from The Century Dictionary.

• noun The middle germinal layer of the three-layered embryo of any metazoic animal, lying between the endoderm and the ectoderm.
• noun In botany, the middle layer of tissue in the shell of the spore-case of an urn-moss.

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

• noun The layer of the blastoderm, between the ectoderm and endoderm; mesoblast. See Illust. of blastoderm and ectoderm.
• noun The middle body layer in some invertebrates.
• noun The middle layer of tissue in some vegetable structures.

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

• noun embryology One of the three tissue layers in the embryo of a metazoan animal. Through embryonic development, it will produce many internal organs of the adult, e.g. muscles, spine and circulatory system.

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

• noun the middle germ layer that develops into muscle and bone and cartilage and blood and connective tissue