The development of isolated and partially separated blastomeres of Amphioxus

Abstract

AbstractBlastomeres of the egg of Amphioxus, separated along the plane of the first cleavage, develop into typical embryos and larvae because all materials of the egg are typically localized in each half. Blastomeres separated along the plane of the second cleavage never develop into typical larvae because the mesodermal crescent is largely lacking in the anterior cells and the chorda‐neural crescent lacking in the posterior ones. Isolated micromeres of the 8‐cell stage form blastulae with ectoderm but no endoderm, they never invaginate; macromeres form gastrulae wiht invaginated endoderm but never develop farther.Partially separated blastomeres develop into partially separated embryos and larvae. Anterior blastomeres of the 4‐cell stage separate more frequently than posterior ones and give rise to embryos and larvae double anteriorly but single posteriorly. Partially separated blastomeres often rotate with respect to each other, but each blastomere invariably preserves its original poles, axes, and germinal localizations. Such rotated blastomeres develop into double embryos and larvae with axes and poles as in the dislocated blastomeres; frequently antero‐posterior and dorso‐ventral axes are completely reversed in the twins.All axes and poles of the future larva are irreversibly determined at or before the first cleavage; complete regulation occurs only in the plane of bilateral symmetry; except for this bilateral regulation, development in Amphioxus, as also in Ascidians, is a mosaic work.