. The physical basis of heredity . Heredity. CEOSSING OVER AND CHROMOSOMES 107 for the ordinary spermatogonia! (or somatic) cell-divi- sions. Each chromosome vesicle begins to show a coiled thread (Fig. 46, c). Each thread next becomes longer and longer (Fig. 46, d) until the whole nucleus is filled with them. One or both ends can often be seen at the "distal pole" of the cell, where deep-staining nucleoli are present. The cells are now in ,the so-called thin thread, or leptotene stages. The threads next come together in pairs beginning at the distal end of the chromosomes (the zygot
. The physical basis of heredity . Heredity. CEOSSING OVER AND CHROMOSOMES 107 for the ordinary spermatogonia! (or somatic) cell-divi- sions. Each chromosome vesicle begins to show a coiled thread (Fig. 46, c). Each thread next becomes longer and longer (Fig. 46, d) until the whole nucleus is filled with them. One or both ends can often be seen at the "distal pole" of the cell, where deep-staining nucleoli are present. The cells are now in ,the so-called thin thread, or leptotene stages. The threads next come together in pairs beginning at the distal end of the chromosomes (the zygotene stage,. FiQ. -Formation of a thick thread after synapsis, a, 6; and the following condensation of a tetrad, c. (After Wenrich.) Fig. 47, a). When the fusion is complete and all the threads are double (Fig. 47, b), the stage is called the thick thread or pachytene stage. There are half as many threads now present as at the beginning. A longitudinal split is present in the chromosome throughout these stages along the line of fusion of the two thin threads. Wenrich identifies the split as the '' primary split.'' Another longitudinal split at right angles to the other one soon appears (Fig. 47, c), thus forming tetrads, each composed of four chromosomes. The tetrads next shorten,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Morgan, Thomas Hunt, 1866-1945. Philadelphia : Lippincott
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