The sorting-out of thymidine-labelled chick hypoblast cells in mixed epiblast--hypoblast aggregates.

Tritium-labelled disaggregated chick hypoblast cells were mixed with non-labelled epiblast cells and vice-versa. The mixtures were allowed to aggregate in a gyratory shaker and were transferred on to a solid culture medium for further incubation. The aggregates were fixed after various incubation times, sectioned and examined for sorting-out. There was already a tendency to sort out after 10 h of incubation, a process which was completed after 25 h. The hypoblast cells formed a continuous layer adjacent to the vitelline membrane, while the epiblast cells moved out to form the upper external layer. The position of the two layers was normal as far as the substrate and external environment are concerned, and reversed in relation to their relative position to the vitelline membrane. The hypoblast cells tended to migrate to the margins of the aggregate. The latter phenomenon seems to parallel the migration of hypoblast cells towards the extra-embryonal area during the formation of the primitive streak.