RESUMEN

1) The development of the head end in the embryo ofSmittia is determined by cytoplasmic components in the anterior region of the egg. Experimental inactivation or removal of these determinants results in the formation of "double abdomens". In embryos with this aberrant metameric pattern, head and anterior segments are replaced by an additional set of posterior segments joined in mirror image symmetry to the original abdomen. Several types of experiment were carried out to determine whether mitochondria act as determinants of the anterior pole in the embryo. 2) The following observations are compatible with a determinative rôle of mitochondria: mitochondria are more abundant in the anterior tip of theSmittia egg which responds most effectively in experiments inducing double abdomens by local UV irradiation. UV irradiation inflicts visible damage upon mitochondria in the irradiated egg region, and significantly decreases the oxygen consumption of the eggs. 3) The following data argue against a determinative rôle of mitochondria: photoreverting treatment after UV irradiation did not cause a significant increase of oxygen uptake. The ATP content of whole eggs and anterior egg halves was significantly increased after UV irradiation of the anterior egg quarter. Localized application of dinitrophenol and ethidium bromide via puncture of the anterior pole, as well as irradiation with intense red light after sensitization of mitochondria with Janus Green B, did not produce double abdomens. 4) After UV irradiation during intravitelline cleavage, photoreverting treatment can be delayed for several hours without diminishing its efficiency; the deadline for photoreversal coincides perfectly with the deadline for UV induction, i.e. with blastoderm formation. This result suggests that the effective targets are metabolically inactive from egg-laying until blastoderm-formation. 5) The results are considered as evidence that mitochondria and the determinants of anterior pole in theSmittia embryo are not identical despite their apparently similar cellular localization.