NUCLEAR SIZES IN RANA MESONEPHROI x MARY GRACE CONNELL 2 /H<>logy Department, The Catholic University of America, Washington, D. C. When Jacobj (1925) devised the now classical caryometric method there was opened up for the cytologist a whole new field in population dynamics. Jacobj demonstrated that for any given organ the nuclear population showed considerable variation in size from one individual nucleus to another. By plotting frequency distribution curves of nuclear volumes he obtained evidence to show that the volumes increased discontinuously according to a logical pattern. The peaks of the curve corresponded to nuclear volumes which when arranged in series gave the geometric progression 1 : 2 : 4 : 8. Nuclear class series have since been described for a wide range of both invertebrate and vertebrate tissues. Caryometry has been used extensively in investigations of ploidy, of endomitotic growth, and of the interphasic growth of nuclei in a dividing tissue. The concept that nuclear size is a function of ploidy has proved fruitful in the study of ploidy in amphibians (cf. Fankhauser, 1945; Gallien, 1953). This idea was used ad-vantageously in the interpretation of polymodal curves obtained from nuclear volume data derived from studies of the kidneys of frogs : it was indicated that polysomaty may occur in this organ (Dawson, 1948; Schreiber and Melucci, 1949). Poly-somaty or endopolyploidy is understood in this paper as that condition existing in a normal diploid somatic tissue in which there is a certain percentage of polyploid cells and/or polytene chromosomes. Furthermore, the concept that nuclear size is a function of chromosomal reduplication has been helpful in the interpretation of data having to do with interphasic growth of nuclei in a dividing tissue. Nuclear class series indicative of a mitotic cycle have been described in Ambystoma larvae (Swift, 1950) and in Rana pipiens embryos (Sze, 1953). Both of these investiga-tions were primarily concerned, not with relationships in size, but with the photo-metric determination of amounts of desoxyribose nucleic acid (DNA) in interphasic nuclei. The introduction into cytology of photometric techniques has renewed interest in the caryometric interpretations of nuclear sizes. Nuclear size as a reflection of ploidy relates importantly to amphibian develop-ment both in regard to gross morphology and in regard to tissue differentiation (Fankhauser, 1945; Gallien, 1953). Also it has been shown in certain molds that both cell size and nuclear size changes may accompany morphogenesis (Bonner, 1957). Furthermore, it has been hypothesized that DNA may show a slight de-crease with the progressive differentiation of certain R. pipiens tadpole tissues (Moore. 1952). Also of importance is the fact that nuclear size may be related to the degree of functional activity of the cells in question. For instance, spinal 1 A paper submitted to the faculty of the Graduate School of Arts and Sciences of the Catholic University of America in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 2 Present address : College of New Rochelle, New Rochelle, New York. 208
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