. The physical basis of heredity . ach half is also split spindle appears, and the cells divide (Fig. 19, f,g,h).In each cell the chromosomes show indications of passinginto a resting stage, as happens after all ordinary celldivisions, but before this change has gone very far a newspindle appears (Fig. 19, i), and preparations for anotherdivision are rapidly made. The new division completesthe maturation of the sperm-cells (Fig. 19, j, kj). Eachof the four cells resulting from the original sperm-mother-cell differentiates into a spermatozoon. In one of the salamanders, Batrocoseps
. The physical basis of heredity . ach half is also split spindle appears, and the cells divide (Fig. 19, f,g,h).In each cell the chromosomes show indications of passinginto a resting stage, as happens after all ordinary celldivisions, but before this change has gone very far a newspindle appears (Fig. 19, i), and preparations for anotherdivision are rapidly made. The new division completesthe maturation of the sperm-cells (Fig. 19, j, kj). Eachof the four cells resulting from the original sperm-mother-cell differentiates into a spermatozoon. In one of the salamanders, Batrocoseps, the matura-tion stages of the male are particularly well shown. Theessential stages in synizesis are shown in Fig. 20, a-das worked out by Janssens. These stages are essentiallythe same as those of Tomopteris. During the early multi-plication stages the cells of the future testes divide by MECHANISM IN SEOREdATION 47 the ordinary mitotic process. The cells then pass into thesynaptic stage (Fig. 20, a-d). As the chromosomes begin. FiQ. 19.—Thin-thread stage of Tomopteris spermatocyte, a-d; tetrads, e; first sperma-tocyte division, f-i; second spermatocyte division,;-^ (After Schreiner). to emerge as thin threads, it is found in Batracoseps thattheir ends are all pointed towards one pole (Fig. 20, d).This is the same pole as that towards which the two ends 48 PHYSICAL BASIS OF HEREDITY of each V-shaped chromosome pointed as the cell wentinto the resting stage. It appears> then that the chromo-somes not only retain their original orientation, but thatthe ends of homologous chromosomes have already come
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectheredit, bookyear1919
