Karyomeres in early cleavage embryos of ophryotrocha labronica lagreca and bacci

Summary 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.This work has been supported by the Swedish Natural Science Research Council and Kungliga Fysiografiska Sällskapet, Lund.The excellent technical assistance of Mrs Annagreta Petersen and Mrs Lena Olsson is gratefully acknowledged.