REGULATION AND THE MODIFICATION OF AXIAL PROPERTIES IN PARTIAL EMBRYOS OF THE NEMERTEAN, CEREBRATULUS-LACTEUS

Embryos acquire axial properties (e.g., the animal-vegetal, dorsoventr al and bilateral axes) at various times over the course of their norma l developmental programs. In the spiral-cleaving nemertean, Cerebratul us lacteus, lineage tracing studies have shown that the dorsoventral a xis is set up prior to the first cleavage division; however, blastomer es isolated at the two-cell stage will regulate to form apparently per fect, miniature pilidium larvae. We have examined the nature of axial specification in this organism by determining whether partial embryos retain the original embryonic/larval axial properties of the intact em bryo, or whether new axial relationships are generated as a consequenc e of the regulatory process. Single blastomeres in two-cell stage embr yos were injected with lineage tracer, and were then bisected along th e second cleavage plane at the four-cell stage. Thus, the relationship between the plane of the first cleavage division and various developm ental axes could be followed throughout development in the ''half-embr yos''. While some embryo fragments appear to retain their original ani mal-vegetal and dorsoventral axes, many fragments generate novel axial properties. These results indicate that axial properties set up and u sed during normal development in C. lacteus can be completely reorgani zed during the course of regulation. While certain embryonic axes, suc h as the animal-vegetal and dorsoventral axes, appear to be set up pri or to first cleavage, these axes and associated cell fates are not irr eversibly fixed until later stages of development in normal intact emb ryos. In C. lacteus, the process whereby these properties are ultimate ly determined is apparently controlled by complex sets of cell-cell in teractions.