. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. VI FUNCTION OF SYNDESIS During their apposition exchange of corresponding factors takes j. and the chromosomes after separation may be constituted in vant ways, , AbCdE and aBcDe ; or abCDe and ABcdE, etc., etc. By this means the Mendehan inheritance of any number of separate characters can be accounted for. The necessity of assuming the interchangeabihty between homologous chromosomes of the chromosome components makes it highly desirable to determine whether parasyndesis is or is not of general occurrence. Paras
. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. VI FUNCTION OF SYNDESIS During their apposition exchange of corresponding factors takes j. and the chromosomes after separation may be constituted in vant ways, , AbCdE and aBcDe ; or abCDe and ABcdE, etc., etc. By this means the Mendehan inheritance of any number of separate characters can be accounted for. The necessity of assuming the interchangeabihty between homologous chromosomes of the chromosome components makes it highly desirable to determine whether parasyndesis is or is not of general occurrence. Parasyndesis obviously offers a favourable op- portunity for the mutual exchange between con- jugating chromosomes of their elements which, as we have seen, are arranged in linear series — and indeed at this stage the corresponding chromomeres of the two chromosomes are often most regularly and con- spicuously in close ap- position (Fig. 77). The evidence for parasyn- desis was discussed in Chapter II., and its general occurrence provisionally accepted. \Ve have moreover Examples of the correspondence between the chromomeres in homo- ' _ logons chromosomes during syndesis. A, Spinax niger ,£ ; B, Myxine direct reason for believ- glulmosa i}. (A and B, afterSchreiners, ., 1906.) C, Dyliscus mar- ginalis S (after Henderson, , 1907). ing that the function of syndesis is not merely that of bringing the members of homologous pairs into apposition so as to effect their sorting out into different daughter nuclei at meiosis. For syndesis often takes place months or years (mammalian oocytes) before the reduction division, and is frequently followed by complete separation of the ex-syndetic chromosomes (many oogeneses, spermatogenesis of Lepidosiren, etc.). Between syndesis and metaphase I. the chromosomes may even undergo metamorphoses as great as those undergone in the resting nucleus (most cases of oogenesis). The separated homologous chromosomes then pair again in prophase I
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