The cell in development and inheritance . es referred to above are nearly related to those thatoccur in the so-called heterotypical mitosis. Under this name Flem-ming (87) first described a peculiar modification of the division of thechromosomes that has since been shown to be of very great impor-tance in the early history of the germ-cells, though it is not confinedto them. In this form the chromosomes split at an early period, butthe halves remain united by their ends. Each double chromosomethen opens out to form a closed ring (Fig. 37), which by its mode oforigin is shown to represent two d
The cell in development and inheritance . es referred to above are nearly related to those thatoccur in the so-called heterotypical mitosis. Under this name Flem-ming (87) first described a peculiar modification of the division of thechromosomes that has since been shown to be of very great impor-tance in the early history of the germ-cells, though it is not confinedto them. In this form the chromosomes split at an early period, butthe halves remain united by their ends. Each double chromosomethen opens out to form a closed ring (Fig. 37), which by its mode oforigin is shown to represent two daughter-chromosomes, each forminghalf of the ring, united by their ends. The ring finally breaks in twoto form two U-shaped chromosomes which diverge to opposite poles DETAILS OF MITOSIS S7 of the spindle as usual. As will be shown in Chapter V.,the divisionsby which the germ-cells are matured are in many cases of this type;but the primary rings here in many cases represent not two but fourchromosomes, into which they afterward break Fig- 37-~ Heterotypical mitosis in spermatocytes of the salamander. [Flemming.]A. Prophase, chromosomes in the form of scattered rings, each of which represents U\odaughter-chromosomes joined end to end. B. The rings ranged about the equator of the spindleand dividing; the swellings indicate the ends of the chromosomes. C. The same viewed from thespindle-pole. D. Diagram (Hermann) showing the central spindle, asters, and centrosomes, andthe contractile mantle-fibres attached to the rings (one of the latter dividing). 2. Bivalent and Plurivalent Chromosomes The last paragraph leads to the consideration of certain varia-tions in the number of the chromosomes. Boveri discovered that thespecies Ascaris megalocephala comprises two varieties which differ inno visible respect save in the number of chromosomes, the germ-nucleiof one form ( variety bivalens of Hertwig) having two chromosomes, 88 CELL-DIVISION while in the other form ( variety univalens
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