During the sixth day, the size of the blastodermic vesicles increase, partly as a result of flattening of the roof cells, and partly as a result of re-arranging and flattening of the cells constituting the floor of the vesicle. In the rat, the loss of the zona pellucida is probably the last event that precedes the beginning of implantation. The shedding of the zona pellucida depends on the stage of development of the ovum ( Dickmann and Noyes, 1961), and this is important because it marks the beginning of the first direct contact between the embryonic and maternal tissues ( Davies and Hesseldahy, 1971).
The blastocysts shed their zonae during the afternoon of day 5 in the uterus before implantation ( Dickmann and Noyes, 1961 Enders, 1971 Lundkvist and Nilsson, 1984) when the pregnant rat is under the influence of both progesterone and estrogen Fig. The trophoblast is continuous, with a massing of epithelial cells at one pole of the blastocyst, the inner cell mass, from which the embryo and its immediate membranes are developed (amnion, yolk sac, allantois, and mesodermal components of the placenta). The trophectoderm cells have descendants only in placental structures (chorion, ectoplacental cone, and trophoblast giant cells). It consists of a hollow sphere of cuboidal trophoblast cells with their apical surface facing on the zona pellucida ( Schlafke and Enders, 1968), with an inner eccentrically placed group of large (7–12 µm), roughly spherical, and loosely associated inner cell mass cells, which occupies about one-quarter of the volume of the sphere. The blastocyst is a roughly spherical epithelial structure that ranges in size from 60 to 85 µm and is surrounded by the 2.5 to 3-µm-thick zona pellucida.
Formation of the blastocoele by cavitation of morula masses composed of 30 to 32 cells begins mid day 5 ( Huber, 1915a).
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