Xenopus mesoderm induction: evidence for early size control and partial autonomy for pattern development by onset of gastrulation

Experiments are described that examine the state of organisation of the presumptive mesoderm and ectoderm of the Xenopus embryo at stages up to the onset of gastrulation. It is shown that a process during blastula stages, establishing the normal proportions in which this cell population is partitioned to found the two outer ‘germ layers’, has a positive regulative property. An operation has been performed to excise the yolky endodermal core, at the beginning of gastrulation, leaving only the presumptive territories of mesoderm, neural tissue, epidermis and supra-blastoporal endoderm. This reveals that by this time a stable capacity exists within the induced tissue to express the craniocaudal sequence of the normal pattern, including the proper numbers of somite segments. The mediolateral organisation of such body patterns is however abnormal. The relevance of the observations to understanding mechanisms of axial pattern control is discussed.