As shown previously by several authors, cycloheximide inhibits progesterone‐induced maturation. However, normal maturation can be obtained, in Xenopus, after a 5 h treatment with cycloheximide (10–20 μ/ml), followed by extensive washing. Under these conditions, protein synthesis is still 50% inhibited. When X.laevis oocytes are continuously treated with a progesterone and cycloheximide mixture, they undergo a degenerative process which has been termed ‘pseudomaturation’: the germinal vesicle membrane breaks down, but the chromosomes do not condense and the nucleoli do not disappear completely. Progesterone‐induced maturation was not arrested by fusidic acid in Rana pipiens, even after micro‐injection. Actinomycin D (10–20 μg/ml) speeds up maturation in both R. pipiens and X. laevis. Microinjection of α‐amanitin into control R. pipiens oocytes does induce maturation in a few cases. It is concluded that one of the effects of progesterone might be a repression of RNA synthesis. Successive treatments with cycloheximide and actinomycin D failed to induce maturation, but often produced ‘pseudomaturation’ in X. laevis. Injection of a cytoplasmic extract (centrifuged homogenate) from X. laevis eggs which have undergone maturation into recipient oocytes of the same species induces ‘pseudomaturation’ and strongly inhibits protein synthesis. If the membranes which surround the egg are eliminated before homogenization, true maturation is obtained. The membranous material apparently releases factors which exert a negative effect on maturation and protein synthesis. Homogenates from eggs which have not been treated with progesterone do not, after injection, induce ‘pseudomaturation’ and have little effect on protein synthesis. In contrast with the findings of Ecker and Smith (1971)11 on R. pipiens, protein synthesis in X. laevis, after a short stimulation, drops considerably (50%) during progesterone‐induced maturation.

The ability of various steroids to induce maturation of Xenopus laevis oocytes after incubation or injection has been studied. Only two of these hormones, hydrocortisone and testosterone, are active by injection as well as by incubation. Progesterone can induce maturation only when administered from the outside medium, which suggests the presence of a receptor in or near the membrane. This progesterone‐receptor has been localized in the melanosomes, and its properties and affinity for other steroids have been studied. The morphological and biochemical effects of the injection of the progesterone‐receptor complex have been studied.

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The formation of an active maturation-promoting factor and a strong burst of protein phosphorylation occur together at germinal vesicle breakdown in Xmopus oocytes undergoing maturation after progesterone treatment. Here we show that one of the sites of this phosphorylation is the 403 ribosomal protein S6. This observation is discussed in relation to the known presence of a maturation-promoting factor in somatic cells during the Gz-M transition and to the incrcase in S6 phosphorylation observed in cells treated with mitogenic stimuli.M at u r a t ion ( or re1 rase of the mci o t i c bl oc k) of amphi bi a n oocytes is the last step of a process initiated in the ovary several months before ; this phenomenon, which in physiological conditions results from a pituitary stimulus, can also be studied in virrw: it is then usually induced by a treatment of the isolated full-grown oocytes with progesterone (for a recent revicw, sec 111

When the supern atan t of a cen trifuged homogen ate from progesteron etreated oocytes is injected into Xenopus full grown oocytes, an in complète and abnormal maturation called "pseudomaturation" occurs. The first sign of pseudomaturation is observed in the cortical région: the plasma membrane becomes lobulated and pièces of cortical cytoplasm are shed into the médium. The most remark able ultrastructural characteristic of pseu domaturation is the hyperdevelopment of the internai membranous System, characterized in particular by the extensive growth of the endoplasmic reti culum, the maintenance of numerous annulate lamellae, unusually slow to disappear. The pores of the latter are often aligned and characteristicaliy united by columns of partially fibrillar materiai. Before the break down of the germinal vesicie, sheets of fibrillar materiai appear at intervais in the cytoplasm, mainly around the nuclear membrane. The basai part of the nuclear membrane undergoes extensive folding before break down of the germinal vesicie occurs; the nuclear membrane usually ruptures near the apical pôle. The main altération in the nucleus is a rapid condensation of the fibrillar core of the numerous nucleoli; the fibrillar and the granular parts of the nu cleoli segregate; they are often found associated with bundles of 340 Â micro tubules. Chromosome condensation is never observed. Several attempts have been made to analyze the mechanisms of amphibian oocyte maturation, in particular hormonal stimulation and the rôles played, respectively, by the nucleus and the cytoplasm (75, 20). Maturation can be induced, in full grown Rana pipiens or Xenopus laevis oocytes, by addition of progestérone to the médium (i, 19). According to Masui and Mark ert (75), maturation can also be induced in large Rana pipiens oocytes by injecting cytoplasm tak en from oocytes which have undergone maturation after progestérone treatment. The présence of a maturation