CURRENT LITERATURE.
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among the pollen grains, but this seems to be nothing more than the occa- sional accidental dislocation or rupture of cells caused by the shriveling ; it is in no sense a tapetal plasmodium.
The author also made incidental observations on the character and development of the poilen grains, and on the structure and mechanism of dehiscence of the anthers.
Sharp, Lester W.— Somatic chromosomes in Tradescantia. Amer . Jour. Bot. 8 : 305-817. 2. pis. 1920.
The details of mitosis are still very incompletely known. Exactly what happens during the reconstruction of the nuclei at telophase ; the condition of the chromatin during the interphase ; the method of reformation of chromosomes from the interphase reticulum ; the time and method of formation of the propbase longitudinal split ; and even the question of the reality of the linin framework of the chromosomes— these all are problems of modern cytology.
The author has concluded previously, following Gregoire, that at telophase the chromosomes undergo a progressive vacuoiation which gradually transforms them into reticular structures, which all together constitute the chromatin network of the resting nucleus. At the next mitosis these reticulated chromosomes gradually separate from each other and condense into the prophase chromosomes. Miss Digby (on the arches porial and meiotic mitoses of Osmunda. Ann. Bot. 33 : 135—370. 1919), among others, has recently produced strong evidence supporting the view that the split observed in the heterotype prophase is the reappearance of a split developed during the previous somatic telophase. The present Study “ was undertaken not only to test the writer's position with respect to the time of chromosome splitting in somatic mitoses, but also, by determining more precisely the nature of the transformation of the chromosomes in the somatic telophase, to ascertain to what extent, if at all, this transformation will aid in the interpretation of the heterotype prophase.”
When the chromosomes begin to separate from one another during teleophase, they remain connected by thin strands of chromatic substance. At the same time minute vacuoles appear in the chromosomes, not only along the midline but also along the periphery, with no regular arrangement. It is this vacuoiation of the chromosomes that has been interpreted by some other workers as a definite longitudinal split ; but transverse sections of the chromosomes may show 3 or 4 vacuoles lying side by side, So that the chromosomes “can no more be called ‘double’ than triple or quadruple.’* Tacuolation continues till the outline of each chromosome is nearly or quite lost in an irregular network. This resting reticulum is regarded as made up entirely of chromatin, or perhaps more accurately, of a very fluid chromatic substance free to diffuse about through another material which composes the framework; but it cannot be interpreted as chromatin granules strung on linin threads.
The prophase of the next mitosis is largely a reversal of the process of vacuoiation. The chromatin begins to condense, and the anastomoses to pull apart. As condensation proceeds, some of the vacuoles open to the exterior, leaving the chromosomes as irregular zigzag threads of uneven thickness,
