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. [Illustration: 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: =From the ectoderm= arises the skin with its appendages, and the salivary and mammary glands; the nasal passages, upper part of the pharynx and the anus; the crystalline lens, the external ear, the entire nervous system, the sense organs and, in part, the fetal membranes. =From the mesoderm= are derived the urinary and reproductive organs; the muscles, bones, and connective tissues and the circulatory systems. =From the entoderm= are developed the alimentary canal, the thymus, thyroid, liver, lungs, pancreas, bladder and the various small glands and tubules.