Fig. 4 a-o: Meiosis in Brachaspis collinus. (All photographs are of acetic-orcein preparations, x 1000.) (a.) Leptotene: The chromosomes appear as a mass of single threads. The X-chromosome is positively heteropycnotic (arrow). (b.) Zygotene: The nucleus has increased in volume and homologous chromosomes have paired. The X-chromosome is still positively heterochromatic (arrow). (c.) Pachytene: Homologues are separating and each can be seen to consist of two chromatids which are joined at chiasmata, the points of cross-over between non-sister chromatids. (c′.) Interpretation of 4c. Each line represents a chromatid. The eleven bivalents can be individually identified (classification is given). (d.) Mid-diplotene: The bivalents have contracted somewhat and chiasmata are more obvious. (d′.) Interpretation of 4d. The position of the centromeres is indicated by small black circles and the chiasmata by arrows. (e.) Early diakinesis: The bivalents have contracted further and some are beginning to become heterochromatic. (e′.) Interpretation of 4e. (f.) Metaphase I: The bivalents have contracted fully and the autosomes are now heterochromatic. The X is negatively heteropycnotic (arrow). (g.) Anaphase I: Separation of the homologous chromosomes each of which consists of two chromatids joined by the terminal centromere giving a ‘v’ appearance. The X-chromosome has segregated undivided to one pole (arrow). (h.) Telophase I: The ‘v-’ shape of the chromosomes is still apparent. The chromosome number at each pole is n = 11 plus X at one of them. (i.) Interkinesis: A short stage between the two divisions of meiosis. Note that only one cell has an X-chromosome (arrow). (j.) Prophase II: The chromosomes are distinguishable once more and are heterochromatic. (The photographs of meiosis II show the division of only one of the cells formed after meiosis I). (k.) Metaphase II: Chromatids of each chromosome are distinguishable and are still joined at their centromeres (arrows). The chromosome number is n = 11 + X. (l.) Anaphase II: The centromeres have divided and the chromatids have separated to opposite poles. The X-chromosome has also divided. (Some of the chromosomes in this photograph are slightly out of focus.) (m-o.) Spermiogenesis: Differentiation of spermatid nuclei (4m) into spermatids (4n-o). Spermatid nuclei with (arrows) and without an X-chromosome can be seen.