Dynamics of the control of body pattern in the development of Xenopus laevis: IV. Timing and pattern in the development of twinned bodies after reorientation of eggs in gravity

The mesendodermal anatomy of twinned larval axes is described in relation to the normal single pattern, when twinning has been caused by experimental tilting of eggs before first cleavage. The formation of two origins for gastrulation movements (dorsal lips) and their relatively rapid spread and coalescence to give a circular blastopore, is a predictor of twin formation in individual embryos after treatment. The anatomy of twins where development has been disturbed from the outset in this way is appreciably different from that induced by the later operation of second dorsal lip implantation. It is also variable in a systematic way. The total sizes of cellular allocations to individual notochords and prechordal head patterns are enhanced above normal if they arise relatively close together in the tissue, but significantly reduced if they arise far apart. These and other features of twinned patterns due to precleavage disturbance are discussed in terms of what they might indicate about the physicochemical nature of the body positional system. The results confirm that by a variety of rather simple, nonsurgical manipulations the relative amounts of territory in the egg devoted to different parts of the body can be greatly influenced.