Theophilus S. Painter

Endomitosis in the nurse cells of the ovary of drosophila melanogaster

1. Nurse cells in the ovary ofD. melanogaster show a range in nuclear diameters of from 5μ to 40μ. This indicates that the volume of the smallest nuclei must have been doubled 8 times to produce the largest nuclei which are therefore at least 512-ploid. 2. In nuclei with the same diameters we find evidence of a series of nuclear changes paralleling ordinary mitosis from a diffuse resting stage to a late prophase condition of the chromosomes. No equitorial plate is formed, nor is there an anaphase movement of the chromosomes, and throughout the cycle the nuclear wall remains in-tact.Geitlers term “endomitosis” is used to describe this inner division cycle of the chromosomes. 3. A striking feature of the endomitotic cycles, inDrosophila ovaries, is the tendency for the chromatids to collect around separate centers in what correspond to early and late prophase stages. In the latter the association is most intimate. 4. In the earlier endomitotic cycles, the association between chromatids is extremely close, and in the late prophase dense chromosome-like aggregates of eight threads are found. In later cycles the association, is less intimate. 5. Five division cycles are figured but from volume considerations, it is concluded that eight must occur to form the largest nuclei, if, as seems very probable, with each division cycle there is a doubling of the nuclear volume. Nurse cells in the ovary ofD. melanogaster show a range in nuclear diameters of from 5μ to 40μ. This indicates that the volume of the smallest nuclei must have been doubled 8 times to produce the largest nuclei which are therefore at least 512-ploid. In nuclei with the same diameters we find evidence of a series of nuclear changes paralleling ordinary mitosis from a diffuse resting stage to a late prophase condition of the chromosomes. No equitorial plate is formed, nor is there an anaphase movement of the chromosomes, and throughout the cycle the nuclear wall remains in-tact.Geitlers term “endomitosis” is used to describe this inner division cycle of the chromosomes. A striking feature of the endomitotic cycles, inDrosophila ovaries, is the tendency for the chromatids to collect around separate centers in what correspond to early and late prophase stages. In the latter the association is most intimate. In the earlier endomitotic cycles, the association between chromatids is extremely close, and in the late prophase dense chromosome-like aggregates of eight threads are found. In later cycles the association, is less intimate. Five division cycles are figured but from volume considerations, it is concluded that eight must occur to form the largest nuclei, if, as seems very probable, with each division cycle there is a doubling of the nuclear volume.

Cell Growth and Nucleic Acids in the Pollen of Rhoeo Discolor

1. A cytoplasmic basophily, which is removed when cells are treated with a ribonuclease enzyme, has been used as a means of identifying ribose nucleic acids and of following their distribution in the cells of the anthers of Rhoeo, and to a lesser extent in the pistil of the flower. Rapid cell growth is invariably accompanied by large amounts of ribonucleic acid in the cytoplasm and heavy deposits of chromatin in nuclei. 2. In young anthers the cytoplasm of both tapetal and sporogenous tissues is extremely rich in ribonucleic acid. As the pollen mother cells prepare for the meiotic divisions, the ribonucleic acid disappears from the cytoplasm, presumably being passed into the nucleus and changed into desoxyribose nucleic acid (chromatin). Initially the cytoplasm of microspores is devoid of basophilic material, but as they grow into mature pollen grains large amounts of ribonucleic acids accumulate in the cytoplasm. 3. The growth of tapetal cells is accompanied by one or more endomitotic nuclear division cycles. The tapetal cells, which are rich in cytoplasmic nucleic acid at the time of cytolysis, are the source of the ribonucleic acid which accumulates in the maturing pollen grains. 4. There is a similarity between microspore growth at the expense of tapetal cells and growth of oocytes in animals through the agency of nurse cells. Large amounts of ribonucleic acid are stored in mature pollen grains and used in the extremely rapid synthesis of new materials which accompanies pollen tube growth.

The effect of carbon dioxide on the eggs of Ascaris

The undivided eggs of Ascaris megalocephala (var. bivalens) were kept in an atmosphere of carbon dioxide for three months. On the removal of the eggs from the gas, a few smears were allowed to undergo full development. Only about one third of the embryo, were normal, the remainder being either masses of disorganized cells, or embryos in which the posterior end was differentiated, together with the primordial germ cells. The problem was to determine the causes of the abnormalities. Eggs were preserved in all stages, stained and mounted in toto. Two distinct and independent causes were found for the abnormal development. The first of these was the fusion of the chromosomes in the equatorial plate phase of the dividing S1-blastomere. (This is the “Ur-somatic” cell in which the diminution process takes place.) The fusion resulted in one of two things: (a) When the fusion involved the greater part of all the chromosomes, the blastomere did not divide. At the next division cycle, a tetraster appeared with eight chromosomes (or their equivalents in small “diminished” chromosomes) lying in the spindles. The tetraster divided very irregularly and the result was the total disorganization of the cells of the ectoderm. Such eggs gave rise to embryos which failed to invaginate. (b) When the fusion involved the ends of the chromosomes only, then division took place, but the chromatin was unequally distributed to the two daughter blastomeres, A and B. This led to an upsetting of the cleavage rhythm of these two cells, the blastomere with less chromatin dividing more rapidly than its mate. The P1 blastomere (the cell which gives rise to the cells of the entoderm, mesoderm, the stomadeum cells, and the primordial germ cells) and its derivatives divide normally throughout development.

A Wing Mutation in Piophila casei

The positive contribution of these matters to the selection problem is to enable us to see the important role played by Mendelisni in the effectiveness of selection. Hereditary variations, such as give rise to the multiple allelomorphs and multiple modifying factors, occur in some organisms rather infrequently, as measured by the time scale of human happenings. If there were no interchange of factors among individuals and stocks, it would take a long time to obtain in one individual all the six diluters of the eosin color of the Drosophila eye; one arises in one illdividual, another in another. But by selective crossbreeding it is possible to bring together into one stock all the modifiers that have been produced in diverse stocks. Mendelism acts as a tremendous accelerator to the effectiveness of selection.