The ventral folding of the caudal extremity of the embryonic disk.
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 tail fold
Historical Examples of tail fold
There is, however, not the slightest indication of a tail-fold, so that there is no inclosed hindgut at all.Development of the Digestive Canal of the American Alligator
Albert M. Reese