Studies on synapsis. I.—Oogenesis in the hymenoptera

Abstract

For establishing a correlation between genetic and cytological phenomena, the Hymenoptera furnish a series of forms which offer fertile possibilities; the production of males from eggs which segment without fertilisation makes it possible to state definitely the point in the germ cycle at which the determination of sex is effected. While, however, much work has been done from the cytological standpoint in connection with the group, our knowledge is still inadequate with respect to those species which produce females as well as males by parthenogenesis. What little is known of this subject has been derived exclusively from a study of the polar and cleavage mitoses; and in view of the importance of treating the germ cell cycle as a whole, and also of the unfavourable material which heavily-yolked ova necessarily afford, it seemed desirable to investigate oogenesis in typical and completely agamic Hymenoptera, more especially because data relating to the maturation prophases of the female germ cells in insects are much less numerous than in the case of spermatogenesis. The original purpose of the author in pursuing the present investigation arose out of previous work on nuclear phenomena in the late ovarian oocytes ofNeuroterus, which yielded indications of post synaptic syndesis. Agar has described a second conjugation of chromosomes after a temporary dissociation of univalents inLepidosiren, unconfirmed hitherto in any other animals; and since the demonstration of such an occurrence in the oogenesis of insects might well provide a basis of reconciliation for the rival hypotheses of parasyndesis and metasyndesis (telosynapsis), the synaptic phase in the oocytes of the Hymenoptera was selected for the first of the present series of studies. In addition to the two-fold problem already stated, there have arisen several subsidiary questions in the course of the investigation, notably the peculiar character of the mitotic figure during ookinesis in certain families, and the significance of certain bodies characteristic of the Hymenopteran egg, namely, the so-called germ cell determinants and the remarkable secondary nuclei. Wherever possible I have avoided the use of sublimate fixatives employed by previous workers. Bouin gave excellent results; but the best preparations were obtained by the use of Flemming modified to increase its penetrating power by means of a trace of urea.