Studies on synapsis. III.—The nuclear organisation of the gern cells in Libellula depressa

An impressive array of facts in support of the chromosome hypothesis has been elucidated by the researches of cytologists during the past two decades; nevertheless the paucity of direct objective evidence for the integral continuity of the chromosomes through all stages of nuclear history still presents a barrier to unqualified acceptance of its validity. Such knowledge is not only essential to a convincing presentment of the chromosome hypothesis: it has furthermore a very intimate bearing upon the interpretation of synapsis. The earlier workers, following Flemming, interpreted the prophase and telophase organisation of the nucleus as a continuous spireme: this notion rapidly hardened into a dogma, which not only handicapped further inquiry into the problem of the constitution of the resting nucleus, but proved to be a source of much confusion respecting the significance of the events in the meiotic phase. How far theory even succeeded in overriding fact may be illustrated sufficiently by a quotation from a paper that has exerted a powerful influence on cytological theory. In reference to the spermatogonial prophases of Periplaneta, the authors state correctly, "the method of chromosome formation here depicted presents nothing exactly comparable to the long spireme thread which is figured in so many existing accounts of the premeiotic divisions." After this unequivocal statement the interpretation given to account for the reduction of the chromosomes is that "the spireme threadwork tends to separate out into half as many lengths" as the number of premeiotic chromosomes. Now, it may safely be said that in general (as well as in the case of Periplaneta) recent research has fully justified Bolles Lee in his contention that "ni à la télophase, ni aux prophases, ni à aucun moment de l'existence de ces noyaux, il y aît formation d'un spiréme continu." Yet even recent workers, like Arnold, 1909 (1), Nakahara, 1919 and Harman, 1920 (2), in their inability to account for telosynaptic union by other means, fall back upon the conception of a spireme segmenting into the haploid number of threads as a means of effecting reduction.