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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.
mes′o·der′mal, mes′o·der′mic adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.