principal piece

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a part or portion.
end piece the terminal portion of the tail of the spermatozoon. See illustration at spermatozoon.
middle piece an anterior portion of the tail of the spermatozoon, between the neck and the anulus. See illustration at spermatozoon.
principal piece the main portion of the tail of a spermatozoon, beginning at the anulus and gradually tapering toward the end piece. See illustration at spermatozoon.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

prin·ci·pal piece

the principal part of the spermatozoon, which is about 45 mc;m long and has a characteristic fibrous sheath surrounding the axoneme.
Farlex Partner Medical Dictionary © Farlex 2012
References in periodicals archive ?
Further research revealed that the protein was being lost because it was not being confined to the principal piece of the tail.
The 1-1[micro]m long principal piece follows the middle piece.
Entitled "The Fall," after one of its principal pieces, the show is uniquely understated -- both in terms of its scale (being comprised of three acrylics and two digital prints) and the style of the work on show.

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