pletely normal fashion to blastocyst, when they may be reimplanted into the uterus of a foster mother for further development. Blastomeres of naked eggs during all stages of cleavage tend to remain in contact with their neighbors, though they may be separated with versene (6). The mouse blastocyst exhibits a striking spontaneous "hatching" out of the zona pellucida in vitro (9). One or more spherical projections, each with the wall composed of a single-cell, extend from the thinly stretched

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... hoblast through what appear to be small pores in the zona pellucida. The remainder of the blastocyst then emerges through a widened crack, and the zona is left behind but does not disappear. This is also presumed to occur in vivo prior to implantation. The possibility suggests itself that blastocyst emergence might result simply from mechanical pressure against the zona as the internal cavity expands with fluid. Eggs were therefore observed in culture after the membrane had been stripped at an earlier stage. Lobes were still "extruded" from the surface in a simulation of hatching. It was also noted that the zona of the morula is more easily ruptured by pipetting than in earlier stages. These data suggest that the zona becomes modified in some way during late cleavage and that some active process is involved in the passage of the blastocyst out of the zona. The surfaces of eggs from which the zona pellucida has been removed are considerably stickier than they are when the membrane is still present, and the eggs tend to adhere to each other. The fusion is greatly accelerated at 370C, and this fact has served as the basis of a method for synthesizing genetic mosaics (4, 8). Accidental loss of the zona pellucida when two or more eggs are present together in vivo could therefore lead to formation of mosaic individuals. An example of this in the human might be the XX/XY case described recently by Gartler et cl. (10).