An assay for the induction of erythroid cells from animal caps. (A) One-cell stage embryos are injected with RNA or plasmids. Animal caps are excised at stage 8 and cultured for 2 days under defined conditions and then dispersed by collagenase. The cells are incubated with o-dianisidine and immobilized onto a glass slide for analysis by microscopy. (B) Protein factors can be tested on dispersed animal caps cells. At stage 8, animal caps are excised and dispersed in CMFM. The outer pigmented layer is discarded. The dispersed inner layer cells are incubated with or without factors. After 1 to 2 hours the cells are washed and reaggregated in calcium/magnesium containing media and cultured for 2 days.o-dianisidine cells are detected as described in (A).