horizon

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horizon

 [hŏ-ri´zon]
a specific anatomic stage of embryonic development, of which 23 have been defined, beginning with the unicellular zygote (fertilized egg) and ending 7 to 9 weeks later with the beginning of the fetal stage.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

horizon

any layer of soil that is distinguishable when soil is examined in vertical sections.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005