In all probability, the embryo is still in the morula stage and is about the size of the head of a pin when it reaches the uterus, where it finds that the endometrium has been prepared for its reception by the premenstrual swelling. The mucosa has grown thicker, more velvety and vascular, and its glands have increased in number and activity. The columnar epithelium of the endometrium is replaced by a thick layer of large, vacuolated cells, called decidual cells, and the uterine lining from now on is termed the decidua gravidatis. While the normal uterine mucosa is thin, averaging from 1 to 3 millimetres (0.039 to 0.117 inch) in thickness, it increases to a thickness of about 1 centimetre (⅞ inch) during pregnancy.
Fig. 18.—Diagram of segmenting rabbit’s ovum.
The point at which the embryo attaches itself to this spongy membrane is entirely a matter of chance. It usually rests somewhere in the upper part of the uterine cavity, promptly destroys the minute underlying area of tissue by digestive action and burrows into the decidua. As the margins of the opening thus made meet and fuse above the ovum, it is completely incapsulated in a cavity of its own that has no connection with the uterine cavity. (Fig. [19].)
After this occurrence the decidua consists of three portions: the hypertrophied membrane which lines the uterus as a whole, called the decidua vera, which atrophies during the latter part of pregnancy and is also thrown off in part with the membranes during labor, and later in the uterine discharges; the decidua basalis, or the decidua serotina, is that portion lying directly beneath the embryo which later enters into the formation of the placenta; and the decidua reflexa, which surrounds and covers the buried embryo, consists of the developed and fused margins of the pit in the mucosa, that have grown over the embryo.
Fig. 19.—Ovum about 13 days old, embedded in the decidua. (The Bryce-Teacher ovum from Human Embryology by Keibel and Mall.)
As the cellular activity continues within the morula, fluid appears in the centre with the result that the cells are rearranged and pushed toward the periphery, thus forming a sac. At this stage the embryo is called the blastodermic vesicle.
At one point on the inner surface of this vesicle the cells proliferate and form a mass which is sometimes called the internal cell mass, or embryonic area, and the single layer of cells comprising the remainder of the vesicular wall, the primitive chorion. The cells in the mass are at first disposed in layers, the outer layer being termed the ectoderm; the inner layer the entoderm, while a third layer which appears a little later is called the mesoderm.
Although these three primitive layers of cells have all arisen from the single cell formed by the fused spermatozoon and ovum, they are even now very different in character. The differences steadily increase until finally all of the complex fetal organs and tissues, the membranes, cord and placenta, result from their further specialization and development, as follows: