Karyomeres in early cleavage embryos of ophryotrocha labronica lagreca and bacci

Karyomeres or chromosome vesicles occur regularly at all cell divisions in cleavage embryos ofOphryotrocha labronica up to the 16-cell stage. They are formed as separate units, containing one or several nucleolus-like bodies (NLB) as well as intranuclear annulate lamellae (IAL), but coalesce later into a compound nucleus, in connection with copious blebbing and simultaneous appearance of cytoplasmic annulate lamellae (CAL). Labelling of the early embryos with³H-thymidine revealed marked localization of the synthesized DNA to the karyomere envelope region, whereas³H-uridine incorporation, indicating RNA synthesis, was sparse and notably absent in the NLB. On the other hand the latter structure like the envelopes preferentially incorporated³H-myoinositol, and displayed considerable labelling with³H-leucine. The mechanism and general significance of karyomere formation is discussed with particular attention to the NLB and their possible involvement in nuclear membrane formation.