Microinjected eggs are normally incubated overnight in Ml6 microdrop culture at 37°C in CO₂ until 1400 h. By this time the eggs will have developed as far as the two-cell stage. The eggs are then implanted into surrogate mothers. As the membrane covering the oviduct of the rat is much tougher than the mouse, we normally cut it with a very fine pair of scissors rather than tearing it. For a female rat that had been successfully induced into pseudopregnancy, the infundibulum will be swollen and the opening of the infundibulum can be clearly seen under the 10-20X magnification of the dissecting microscope.