Distribution and size of the cleavage nuclei in fragmented eggs ofAcheta domesticus L. (Orthopteroidea) during early differentiation

Abstract

In the egg ofAcheta domesticus L., peripheral streaming alters the originally equidistant distribution of early (plasmodial) blastoderm nuclei. Nuclei accumulate in the egg region between 10 and 50% of egg length measured from the posterior pole, while in other egg regions the distance between nuclei increases. The crowded nuclei will form the germ anlage proper. As a first step they reduce their volumes considerably. In the present study, whole mounts of eggs separated during cleavage into 2 fragments were scored for nuclei of reduced volume. The results were compared to the capability of similar egg fragments to form complete or partial germ bands.In eggs separated at 30% egg length, most nuclei in the posterior fragment will suffer size reduction and, if development is allowed to proceed, will form complete germ bands. In the anterior fragments, nuclei which normally will participate in head formation are carried backwards by peripheral streaming and accumulate near the rear end of the fragment. However, their volumes do not decrease, and no germ bands parts will be formed in such fragments during further development; instead, the blastoderm is converted exclusively into extraembryonic covers. These results show that the streaming and crowding of nuclei as such are not sufficient to induce size reduction and the switch towards germ band formation. It is concluded that for these changes to occur some additional influence is needed. This should in normal development be exerted by the posterior egg region which was separated from the anterior fragment in the experiment.