Bryce and Teacher[1] the point of entrance was visible as a small gap closed by a mass of fibrin and leucocytes; in the ovum described by Peters[2] the opening was covered by a mushroom-shaped mass of fibrin and blood-clot (Fig. 32), the narrow stalk of which plugged the aperture in the mucous membrane. Soon, however, all trace of the opening is lost and the ovum is then completely surrounded by the uterine mucous membrane.
The structure actively concerned in the process of excavation is the trophoblast of the ovum, which possesses the power of dissolving and absorbing the uterine tissues. The trophoblast proliferates rapidly and forms a network of branching processes which cover the entire ovum and invade and destroy the maternal tissues and open into the maternal bloodvessels, with the result that the spaces n the trophoblastic network are filled with maternal blood; these spaces communicate freely with one another and become greatly distended and form the intervillous space.
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