. The physical basis of heredity . 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
. The physical basis of heredity . 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. -^S ^^^ ^ FiQ. 20.—Synaptic stages and those immediately following in Batracoseps. (After Janssens.) together, or are coming together, as the followingstages show clearly. The union that begins at the ends (Fig. 20, e) grad-ually extends along the length of the chromosomes, which MECHANISM IN SEGREGATION 49 are now in the form of thin threads. At the point wherethe two threads come together (Fig. 20,/) they can oftenbe seen to be shaped like a. Y and, at the point of meeting,the uniting threads are often twisted about each other. The fused part of the united tlireads steadily growsshorter and thicker. They become the condensed pachy-tene threads, and appear as represented in Fig. 20, g. Thethick threads shorten further, and the line of fusionbetween them (or a. new line of cleavage) appears, asseen in Fig. 20, h. It will be noticed also that the raggedoutline that the chromosomes had during the precedingstages is gradually lost, so that they now appear as solidrods or cords, which f
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