. The cell in development and inheritance. Cells. 326 so.]//-: /'A'()/>'/,/•;.)/.v o/-' matocytc-divisions ot the salamander. In leueocytes (Fig. 49) the large persistent aster has at its centre a well-marked radial sphere bounded by a circle of microsomes, as described by Van Beneden, but without division into cortical and medullary zones. The astral rays, however, show indications of other circles of microsomes lying outside the centrosphere. Driiner found that a whole series of such concen- tric circles might exist (in the cell shown in Fig. 156 no less than nine), b
. The cell in development and inheritance. Cells. 326 so.]//-: /'A'()/>'/,/•;.)/.v o/-' matocytc-divisions ot the salamander. In leueocytes (Fig. 49) the large persistent aster has at its centre a well-marked radial sphere bounded by a circle of microsomes, as described by Van Beneden, but without division into cortical and medullary zones. The astral rays, however, show indications of other circles of microsomes lying outside the centrosphere. Driiner found that a whole series of such concen- tric circles might exist (in the cell shown in Fig. 156 no less than nine), but that the innermost two are often especially distinct, so as to mark off a centrosphere composed of a medul- lary and a cortical zone precisely as described by Van Beneden. These observations show conclusively that the centrosphere of the radial type is merely the innermost portion of the aster, which acquires a boundary through the especial development of a ring of microsomes, or other- wise, and which often further acquires an intense staining-capac- ity so as to appear like a centrosome (p. 313). In Tliysanozoon {N?i\\ diOX Stricht) only a single ring of micro- somes exists, and this lies at the boundary between the medullary and cortical zones (Fig. 152, D), the latter differing from the outer region only in the greater delicacy of the rays and their lack of staining-capacity, thus producing a " Heller ; In other cases, no " microsome-circles " exist; but even here a clear zone often surrounds the centrosome { in Physa, t. Kostanecki and Wierzejski), like that seen in the cortical zone of ThysanozooJi. There are some observations indicating that the entosphere (medul- lary zone) may be directly derived from the centrosome (central granule). This is the conclusion reached by Lillie in the case of Unio referred to above, where, during the prophases of the second polar spindle, the central granule enlarges and breaks up into a group of granules
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1906
