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The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
blastulaA hollow sphere formed during the early embryonic development of higher animals—which follows the morular stage of embryogenesis—in which dividing cells line up on the outside and are bathed by fluid within the adjacent cavity (blastocoele). In animals where cleavage involves the whole egg, the blastula consists of a ball of cells with a hollow centre.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005