STAGING OF NEWT BLASTULA EMBRYOS AND FIRST APPEARANCE OF PRIMARY MESODERMAL COMPETENCE

Using the swimbladder of the crusian carp (Carrasius auratus) as an inductor, the first appearance of mesodermal competence in the presumptive ectoderm of the newt (Triturus pyrrhogaster) blastula was investigated. The time course of embryonic development before the gastrula stage was determined by counting the number of surface cells on a 0.25 mm line at the animal pole. Pregastrula embryos with 2-3, 4-5, 6-7 and 7-8 cells roughly correspond to those at 14, 14-12, 8-6 and 4-0 hr before the beginning of gastrulation. Using presumptive ectoderm of the early gastrula stage, 15 min was found to be the minimum time of contact necessary for the realization of induction. The reactivity of the presumptive ectoderm from pregastrula embryos was tested by 30 min contact. Presumptive ectoderm up to the 4-5 cell stage did not react to the inductor. It may become competent within the next 4-8 hr, since the ectoderm from embryos in the 6-7 cell stage was reactive.