Cytochalasin B (CB) was applied to early developmental stages of the egg of the squid Loligo pealei and defects in cleavage and/or organogenesis were produced. If CB was applied in relatively high dosages (2·0 μg/ml) or for long periods (30 min) to embryos in early cleavage the existing furrows irreversibly disappeared. If precleavage embryos were similarly treated the streaming pattern which forms the blastodisc was interrupted and patches of clear cytoplasm appeared on the zygote surface. Short low dosage pulses (0·2 μg/ml for 10 min) produced cleavage effects correlated with the time of pulsing. If pulsed within 30 min of first cleavage the first furrow did not appear in the majority of cases but the second furrow appeared synchronously with the controls and appeared normal in all respects. If pulsed approximately 30 min or longer before cleavage the first furrow usually appeared on time as a weak surface line. Second cleavage appeared normally. However, the asymmetrical pattern of third cleavage was ‘equalized’ and the blastomeres tended to be of equal size. It was concluded that there are subtle cytoplasmic movement patterns, sensitive to CB, which position the nuclei before each cleavage and thereby determine the cleavage pattern.

If embryos were pulsed (0·2 μg/ml for 10 min) at various times during early development, anomalies in differentiation at organogenesis (3–4 days later) were produced which correlated with the time of pulsing. Pulses after germinal layer formation (stage 11) produced no noticeable effects and pulses before the cytoplasmic streaming which produces the blastodisc similarly had very little or no effect. However, pulses during blastodisc formation or early cleavage produced severely affected embryos in which organ displacement, poor tissue and organ differentiation, or organ deficiencies were common. Pulsed embryos were scored and the time of pulsing correlated with the severity of effects. The nuclei of the inductive yolk epithelium were abnormal, frequently being rounded, clumped and/or containing clumped or abnormally diffuse chromatin. It was concluded that the streaming pattern which forms the blastodisc in some way fixes or enhances a pre-existing pattern of developmental information which resides in or at the surface of the egg. Once this pattern is established it is insensitive to CB but it can still be demonstrated by other techniques. This informational pattern apparently influences the nuclei of the yolk epithelium that come to lie in specific regions and controls the expression of their genome so that specific organs are programmed.

This content is only available via PDF.