The decidual reaction constitutes an important part of the response of the uterus to the embryo which it contains; yet in no species has it so far been possible to identify what feature of the embryo stimulates this response. The mouse is a suitable species in which to investigate this point, since the decidual reaction is intense and relatively precocious.
Having established that the distension of the uterus caused by the physical presence of the blastocyst was not responsible for inducing the decidual response (McLaren, 1968), an attempt was made to test the further hypothesis that the loss of the zona pellucida, and the consequent close contact between the vitelline surface and the uterine epithelium, constitutes the inducing stimulus. The time of loss of the zona in mice (Orsini & McLaren, 1967) coincides rather closely with the
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