PATTERN-FORMATION FAILS AFTER BLASTODERM FORMATION BY RAPID CELL-CYCLES IN AN ARTIFICIALLY ACTIVATED INSECT EGG

Oocytes explanted from adult ovaries of the arrhenotokous Hymenopteron Pimpla turionellae remain in an inactive state, because development h as not been initiated by mechanical deformation during natural oviposi tion. However, they could be induced to enter development by injecting cleavage energids into the posterior pole. After lag phases of up to 32 h, the implanted nuclei initiated a normal cleavage process, except that the polarity of its progress was reversed. In other oocytes, the injected energids congregated in a ring-shaped region at the egg surf ace to form a superficial ''nuclear front'', which slowly advanced tow ards the anterior egg pole, thereby successively stimulating portions of the quiescent ooplasm to take part in development. Up to 41 rapid c ell cycles started from that front, each of them with an anaphase wave running backwards into the region already peripherally occupied by nu clei. Thus, the blastoderm was formed extremely metachronously and by rapid obviously biphasic cell cycles, which never occur at the egg sur face during normal cleavage. A germ band, however, was only formed und er the following conditions: (1) that cleavage did not follow the nucl ear front mode, and (2) that ooplasm from the donor's posterior pole w as co-injected with the graft nuclei. We conclude that embryonic diffe rentiation requires some of the events which had been omitted in eggs where development failed, especially the exponential increase of the c ell cycle length, and the activity of some posterior factor(s) during egg activation.