The absence, suppression, or failure to develop one member of a series, such as a finger or vertebra.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
Examples from the Web for excalation
In fact, more or less complete “excalation” of the somite takes place.