THE ESTABLISHMENT OF EMBRYONIC AXIAL PROPERTIES IN THE NEMERTEAN, CEREBRATULUS-LACTEUS

Experiments were performed to determine whether the first cleavage div ision plays a role in setting up the dorsoventral axis in embryos of t he equal-cleaving nemertean Cerebratulus lacteus. Fertilized eggs were compressed to change the orientation of the first cleavage spindle, a nd thus the plane of the first cleavage division. One cell of the resu lting two-celled embryos was then injected with lineage tracer to dete rmine whether the first cleavage plane always maintains its normal rel ationships to the median and frontal planes or whether new relationshi ps (and thus, novel cell lineages) could be created. Many of these com pressed embryos gave rise to normal-appearing pilidium larvae in which the first cleavage plane had taken on various oblique angular relatio nships relative to the plane of bilateral symmetry and the dorsoventra l axis of the larva. These findings indicate that the first cleavage p lane can be dissociated from its normal relationships to these axial p roperties. Thus, the first cleavage division is not causally involved in the establishment of the dorsoventral and bilateral axes. We argue that the dorsoventral axis is specified prior to the first cleavage di vision. (C) 1996 Academic Press, Inc.