Abstract
Part I. 1. Univalent formation in progeny from a diploid branch of a haploid individual ofCrepis capillaris belonging to theX strain is caused through failure of chiasma formation between homologous chromosomes which are shown to be regularly associated at pachytene. 2. A significant difference in the amount of univalent formation is found between two plants, and between two buds on the same plant. 3. Chiasma frequency is reduced in all three kinds of bivalents, and there is an almost direct relation between the amount of chiasma formation and the size of chromosomes. This is shown by the percentage of univalents and the mean number of chiasmata per bivalent. 4. Univalent homologous chromosomes frequently lie in juxtaposition at diakinesis. This is considered to be due to their previous association and coiling at pachytene and early diplotene. Part II. 5. TheA bivalents in a single plant ofCrepis capillaris have a reduced frequency and an abnormal distribution of chiasmata. In bivalents with single interstitial chiasmata, 95 per cent. have the chiasma in a subterminal position at the end of the long arm. Univalents of these chromosomes are occasionally found, and unpaired threads are observed at pachytene. Chiasma formation and possibly pairing are therefore restricted in one region of the chromosome.
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