. The cell in development and inheritance. Cells. 194 REDUCTION OF THE CHROMOSOMES spermatogonium-clivision, which spHt lengthwise during the anaphase and pass into the spermatocyte-nucleus without forming a reticulum. Flemming had observed in 1887 that these daughter-chromosomes split in the anaphase, but could not determine their further history. Vom Rath found that each double daughter-chromosome breaks in two at the apex to form a tetrad, which passes into the ensuing spermatocyte without the intervention of a resting stage.^ It is clear that in such cases the " pseudo-reduction "
. The cell in development and inheritance. Cells. 194 REDUCTION OF THE CHROMOSOMES spermatogonium-clivision, which spHt lengthwise during the anaphase and pass into the spermatocyte-nucleus without forming a reticulum. Flemming had observed in 1887 that these daughter-chromosomes split in the anaphase, but could not determine their further history. Vom Rath found that each double daughter-chromosome breaks in two at the apex to form a tetrad, which passes into the ensuing spermatocyte without the intervention of a resting stage.^ It is clear that in such cases the " pseudo-reduction " must take place at an earlier period than the penultimate generation of cells. In the salamander Flemming ('87) found that the " chromosomes " of the spermatogonia appeared in the reduced number (twelve) in at least three cell-generations preceding the penultimate. Vom Rath ('93). Fig. 97. — Longitudinal section through the ovary of the copepod Canthocamptus. [HACKER.] og. The youngest germ-cells or oogonia (dividing at ^^.-) ; a. upper part of the growth- zone ; oc. oocyte, or growing ovarian egg; ov. fully formed egg, with double chromatin-rods. traced the pseudo-reduction in both sexes back to much earlier stages, not only in the larva;, but even in the embryo (!). This very remark- able discovery showed that the pseudo-reduction miglit appear in the early progenitors of the germ-cells during embryonic life —perhaps even during the cleavage. This conjecture has apparently been substan- tiated by Hacker ("95, 3), who finds that in Cyclops brevicornis the 1 It is certain that these facts do not represent a universal type of maturation, for in Ascaris there is no doubt that a true reticular resting stage occurs in the primary spermato- cytes, and probably also in the germinal vesicle. Hacker found, moreover, that the same species might show differences in this regard; for in Cyclops strenuus the first-laid eggs have no resting stage, the double daughter-chr
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectcells, bookyear1896
