(A) The operating medium drop/PB1 for labeling. The embryo is kept stationary with the holding pipette (1). With the holding pipette the embryo is moved to the edge of the PB1 such that the labeling site is nearest to the medium-oil interface. The labeling pipette (2) is loaded with dye and kept in the oil phase near the medium drop. The labeling pipette and site of labeling is aligned to ensure they are in same focal plane. (B) Move the labeling pipette slowly into the medium drop to ensure that the dye is not leaking from the pipette. (C) When the labeling pipette touches the site of labeling on the embryo carefully expel the dye in the labeling pipette. When an adequate amount of dye has been released onto the embryos, stop further painting by lowering the oil pressure in the labeling pipette and quickly remove the pipette from the medium drop. (D) Successive labeling using double-instrument holder. (1) 7.5-dpc embryo to be labeled. (2) Holding pipette. (3) and (4) Separate droplets of DiI and DiO. (5) Two labeling pipettes filled with either DiI or DiO and mounted on a double-instrument holder which allows for successive deployment of the two pipettes for labeling.