Fig. 3

Mesodermal cells in front of the marginal domain are already allocated to somites. (A) The diagram shows the position of cell pairs (yellow dots) injected at about the same anteroposterior level anterior to the marginal domain (mz) at 100% epiboly. Injections in a given embryo are linked. The graticule used for injections (see Materials and Methods) and the presumed location of the expression domains are projected onto the positions of the injected cells. (B) Fluorescence micrograph of a 100% epiboly embryo immediately after injection of hypoblast cells bilaterally at the same anteroposterior level (arrows). (C) In five cases, single cell injections were made on both sides; labelled cells (arrows) were restricted to the same somite on both sides. (D) In the remaining four embryos, two or more cells were injected on at least one side. In one embryo, labelling was restricted to the same somite on both sides; in the other three, progeny cells were found unilaterally in neighbouring somites; however, in each of these cases labelled cells were also within the same somite on both sides. C and D, same magnification. Scale bars, 100 μm.