Cortical movements in the egg o/Barnea candida (Mollusca, Lamellibranchiatd) studied with the electron microscope

  1. The crenulations of the egg-chorion (‘vitelline membrane’), previously described in Chaetopterus, have been observed occasionally in certain batches of Barnea’, they may be provoked with greater intensity by cold treatment.

  2. In both cases the electron microscope demonstrates that these undulating movements are the result of an elongation of the microvilli, occurring in suc-cessive waves.

  3. This elongation of the microvilli is autonomous and occurs independently of any undulation of the basal plasmalemma.

  4. The tips of the microvilli are firmly embedded in the dense peripheral layer of the chorion, which is, as a result, lifted up and folded by the elongation of the villi.

  5. In eggs which have been cold treated before the expulsion of the first polar body, this expulsion is considerably delayed; under these conditions the undula-tions of the chorion may be observed before and during the formation of the polar body, progressing from the animal towards the vegetative pole.

  6. Those undulations progressing towards the vegetative pole provoke a translation movement of the chorion which accumulates at the vegetative pole. Through this translation there occurs a tangential disrupture at the base of the microvilli; also, the microvilli-chorion complex is thus lifted from the basal plasmolemma resulting in an ‘elevation of the membrane’, a condition which is pathological for the species (Barnea and Chaetopterus) considered in the present work.

  7. The meaning of this cortical activity is discussed. It appears in correlation with physico-chemical changes affecting the entire surface of the egg immediately before the formation of furrows and furrow-like deformations (i.e. polar lobe).

  8. In all cases in which the formation of furrows is either abortive (monasters) or considerably delayed (cold shock), this cortical activity is enhanced.

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