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
EIGHT FIGURESThe clianges in the medullary bone of laying birds offer unusual opportunities for the study of several aspects of bone formation and destruction. Thus the rapid cyclic formation and breakdown of this bone in laying pigeons enabled Bloom, Bloom, and McLeaii ('41) to conclude that the various types of cells of bone are temporary functional states of the same cell. We have continued to study tlie medullary bone of laying birds and have tried to find out whether the cellular transformations are especially rapid in the chicken with her large clutches and extended periods of egg laying.
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