Article révisé par les pairs
Résumé : The effects of cytochalasin B on three different biological systems have been studied with cytological methods. Cytochalasin B stops immediately the cleavage of Tunicate eggs, without preventing nuclear multiplication; however, the mitotic apparatuses are disorganized, as indicated by the presence of cytasters and anastral mitoses. Cytochalasin B produces fusion of the nucleoli and strong condensation of the chromosomes in amphibian oocytes. It does not prevent progesterone induced maturation; but the latter is always abnormal and abortive. Cytochalasin is much more toxic for anucleate than for nucleate fragments of Acetabularia. It reversibly stops cytoplasmic streaming. Abnormalities are much more frequent in anucleate fragments when cytochalasin treated fragments of the algae are returned to normal medium. The significance of these results is discussed.