Deletion‐transplantation experiments on embryos of Fundulus heteroclitus. I. The posterior embryonic shield

Experiments are described in which the differentiative capacity of the posterior embryonic shield of the teleost, Fundulus heteroclitus, was investigated by means of deletion‐transplantation techniques. The posterior embryonic shield, removed from early gastrulae to immediate postgastrulae and grafted into the pericardial cavity of slightly older embryos, exhibits considerable growth and differentiates typical trunk and tail structures involving all germ layers. The donor embryos, in general, develop normal heads to pectoral fin regions. They lack all trunk structures except for gut and some gut derivatives. They frequently differentiate a very small “tail” with unpaired somites. The “tail” sometimes flattens and forms fin rays. Collateral experiments involving the deletion and grafting of various parts of the germ ring provide data in agreement with the conclusion that the posterior embryonic shield supplies the bulk of material for the formation of the trunk and tail in teleosts. The experiments are discussed from the standpoint of their elucidation of gastrulation, trunk and tail formation, embryonic regulation of defects, and endoderm formation in teleosts.