. Cytology, with special reference to the metazoan nucleus. Cells. IV THE SEX CHROMOSOMES 105 consisting of two or more components. These are separate in the somatic or premeiotic nuclei of both sexes, and in all nuclei of the female (with the probable exception of Phylloxera caryaecaulis, see p. 117) ; in the meiotic phase of the male, however, they commonly become associated in a degree varying from merely a more or less close grouping (Acholla) to an actual junction (Syromastes) (Figs. 48, 49, 50). The Y chromosome is always simple, even in those species where the X is compound. The. Fig. 4
. Cytology, with special reference to the metazoan nucleus. Cells. IV THE SEX CHROMOSOMES 105 consisting of two or more components. These are separate in the somatic or premeiotic nuclei of both sexes, and in all nuclei of the female (with the probable exception of Phylloxera caryaecaulis, see p. 117) ; in the meiotic phase of the male, however, they commonly become associated in a degree varying from merely a more or less close grouping (Acholla) to an actual junction (Syromastes) (Figs. 48, 49, 50). The Y chromosome is always simple, even in those species where the X is compound. The. Fig. 49. C I The chromosomes of Syromastes (A, B) and Acholla multispinosa (C-I). (Wilson, , 1909 a, and Payne, , 1910.) In A and B the X chromosomes are blackened. A, 0" diploid group, 20+X-2 = 22 chromosomes ; B, $ diploid group, 20+2X3 = 24 chromosomes ; C, 9 diploid group, 20+2X5 = 30 chromosomes. The three pairs of small X elements are easily seen, the other two pairs are not at this stage distinguishable. D, 0* diploid group, 2o+X5+Y = 26 chromosomes; E, metaphase I., £, polar view. The Y element is characteristically in association with the two largest of the X elements. F, G, anaphase I., side view cut in two sections. The section in F contains the Y and the two large X elements, and that in G the three small X elements. H, I, anaphase II., showing the results of the segregating division. following examples illustrate the range of variation in the constitution of the sex chromosome (Fig. 51) : (A) No Y chromosome present. 1. X Chromosome single (Protenor). 2. X ,, double (Syromastes). 3. X ,, pentad (Ascaris lumbricoides). (B) Simple Y chromosome present. 1. X Chromosome single (Lygaeus, Oncopeltus). 2. X ,, double (Fitchia). 3. X ,, triple (Prionidus). 4. X ,, quadruple (Gelastocoris). 5. X ,, pentad (Acholla multispinosa).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration an
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