. The physical basis of heredity . Fig. 46.—Cells emerging from their last spermatagonial division, a. h\ passing into thesynapsis stage, c, d; (After Wenrich.) CROSSING OVER AND CHROMOSOMES 107 for the ordinary spermatogonia! (or somatic) cell-divi-sions. Each chromosome vesicle begins to show a coiledthread (Fig. 46, c). Each thread next becomes longerand longer (Fig. 46, d) until the whole nucleus is filledwith them. One or both ends can often be seen at thedistal pole of the cell, where deep-staining nucleoliare present. The cells are now in the so-called thinthread, or leptotene stages. T
. The physical basis of heredity . Fig. 46.—Cells emerging from their last spermatagonial division, a. h\ passing into thesynapsis stage, c, d; (After Wenrich.) CROSSING OVER AND CHROMOSOMES 107 for the ordinary spermatogonia! (or somatic) cell-divi-sions. Each chromosome vesicle begins to show a coiledthread (Fig. 46, c). Each thread next becomes longerand longer (Fig. 46, d) until the whole nucleus is filledwith them. One or both ends can often be seen at thedistal pole of the cell, where deep-staining nucleoliare present. The cells are now in the so-called thinthread, or leptotene stages. The threads next come together in pairs beginningat the distal end of the chromosomes (the zygotene stage,. Fia. 47.—Formation of a thick thread after synapsis, a, 6; and the following condensationof a tetrad, c. (After Wenrich.) Fig. 47, a). When the fusion is complete and all thethreads are double (Fig. 47, &), the stage is called thethick thread or pachytene stage. There are half as manythreads now present as at the beginning. A longitudinalsplit is present in the chromosome throughout these stagesalong the line of fusion of the two thin threads. Wenrichidentifies the split as the primary split. Another longitudinal split at right angles to the otherone soon appears (Fig. 47, c), thus forming tetrads, eachcomposed of four chromosomes. The tetrads next shorten, 108 PHYSICAL BASIS OF HEREDITY opening out in various ways to produce fignires lil^e thoseshown in Fig. 47, c. The sex-chromosome (X) that has no mate in thePhrynotettix male, and hence has not conjugated, has only-one longitudinal split (a dyad). The cell, the primaryspermatocyte, with its nucleus next divides. Eleven auto-s
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectheredit, bookyear1919
