Changes in the formation of nuclear membranes and the ultrastructure of chromosomes during the early development of locust eggs (Locusta migratoria).

Observations on the formation of nuclear membranes and of the changing ultrastructural appearance during mitosis in developing eggs ofLocusta migratoria are reported. 1. Nuclear membranes are formed while the mitotic chromosomes are still in a condensed state. How exactly the membranes are formed, is not yet understood. Vesicles which occur in the interchromosomal space might participate in the membrane formation but they might just as well appear simultaneously with the membranes. 2. Depending on the developmental stage of the egg, differences in the way in which the interphase nucleus is built up, are found. In early cleavage, karyomeres are formed from single chromosomes which are individually covered by a double membrane. These karyomeres expand by uncoiling their chromosomal mass and subsequently fuse to form one nucleus. In embryonic cells of the germinal rudiment the double envelope is formed only after the chromosomes have clustered and have formed the telophase figure. 3. Nuclear membranes are formed in close contact with the condensed chromosomes. Only after the membranes have been built up, the chromosomes lose their electron density. Chromosomes in early developmental stages (cleavage) uncoil in karyomeres, although the mitotic cycle may only have reached anaphase. In cells from later stages (germinal rudiment), where the chromosomes cluster before membrane formation occurs, the membrane formation as well as the uncoiling are postponed to telophase. This behavior may be related to changes in the duration of the mitotic cycle, indicating progressive differentiation. 4. During the growth of karyomeres, chromosomes lose most of their electron density. Dense cores of 0.2 Μ in diameter, however, persist in the karyomeres of cleavage energides and may prove the existence of a chromosomal axis in certain mitotic stages. 5. Nucleoli are not found in karyomeres during early cleavage stages. Prenucleoli are observed to form at telophase in the later stages of development. More prenucleoli seem to occur in telophase than do nucleoli in interphase nuclei.