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ONE TEXT FIGURE AND FOUR PLATE'S (SEVEN FIQURES) This is a description of tlie rapid changes observed in the bone formed in the medulla of some of the long bones during the preovulatory, ovulatory, and postovulatory periods of the female pigeon (Bloom, Bloom, Domm and McLean, '40). This work is a part of a general study of bone formation mid destruction and was stimulatecl in particular by the observation by Kyes and Potter ('34) of the development of bone in the niarrow of pigeons with large ovarian follicles. Observations made on laying chickens and ducks and the findings in pigeons injected with male and female sex hormones will be reported separately.Our interest in the reniai-kable bone changes in the marrow of birds in connection with ovulation and egg-laying centers about the general questions of : (1) The relations between bone iiiatrix and bone salt during the deposition and removal of hone, (2) the hone-forming potentialities of reticular cells, and (3) the origin arid fate of the cells associated Fit11 tlie foi-mation and dest1*uction of bone. Because of the rapidity of the changes in the bone during certain stages of the cgg-1 a~i n g cycle, this material offers unusual opportunities for the study of these problems.'This work has been rtided by grants from the nr. Wallace C. and Clara A.X11l)ott Memorial Fund of The University of Chicago, and from the Josiah Macy, Jr. Foundation. 443

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The exceptionally organic-rich rocks of the 1.98 Ga Zaonega Formation deposited in the Onega Basin, NW Russia, have refined our understanding of Earth System evolution during the Paleoproterozoic rise in atmospheric oxygen. These rocks were formed in vent-or seepinfluenced settings contemporaneous with voluminous mafic volcanism and contain strongly 13 C-depleted organic matter. Here we report new isotopic (δ 34 S, Δ 33 S, Δ 36 S, δ 13 Corg) and mineralogical, major element, total sulphur and organic carbon data for the upper part of the Zaonega Formation, which was deposited shortly after the termination of the Lomagundi-Jatuli positive carbon isotope excursion. The data were collected on a recently obtained 102 m drillcore section and show a δ 13 Corg shift from-38‰ to-25‰. Sedimentary sulphides have δ 34 S values typically between +15‰ and +25‰ reflecting closed-system sulphur isotope behaviour