Fig. 1. Experimental set-up for DiI injection shown in transection. (A) Whole 6.0 d.p.c. blastocyst (c) with intact zona pellucida (z) held on a metal ring (r). Embryonic disc (d) with its anterior margin (*) projects above the fluid level. The tip of the injection pipette and the injected dye lie in the space between epiblast (e) and zona pellucida (z), thereby avoiding contact with the hypoblast (h). (B) DiI injection sites just anterior to and within the PGE. Lateral injection sites were grouped symmetrically with regard to the median plane; for simplicity, they are shown only on the left of the embryonic disc and the numbers of injections carried out in corresponding injection sites are accumulated.