At this stage a complete chorion has become formed, and is probably derived from a growth of the mesoblast of the allantois (unaccompanied by the hypoblast) round the whole inner surface of the subzonal membrane. From the whole surface of the chorion there project branched vascular processes, covered by an epithelium. The allantois is without a cavity, but a hypoblastic epithelium is present in the allantoic stalk, through which it does not, however, form a continuous tube. The blood-vessels of the chorion are derived from the usual allantoic arteries and vein. The general condition of the embryo and of its membranes at this period is shewn diagrammatically in fig. 147, 5. Around the embryo is seen the amnion, already separated by a considerable interval from the embryo. The yolk-sack is shewn at _ds_. Relatively to the other parts it is considerably smaller than it was at an earlier stage. The allantoic stalk is shewn at _al_. Both it and the stalk of the yolk-sack are enveloped by the amnion (_am_). The chorion with its vascular processes surrounds the whole embryo. It may be noted that the condition of the chorion at this stage is very similar to that of the normal diffused type of placenta, described in the sequel. While the above changes are taking place in the embryonic membranes, the blastodermic vesicle greatly increases in size, and forms a considerable projection from the upper wall of the uterus. Three regions of the uterine wall, in relation to the blastodermic vesicle, are usually distinguished; and since the superficial parts of all of these are thrown off with the afterbirth, each of them is called a decidua. They are represented at a somewhat later stage in fig. 151. There is (1) the part of the wall reflected over the blastodermic vesicle, called the decidua reflexa (_dr_); (2) the part of the wall forming the area round which the reflexa is inserted, called the decidua serotina (_ds_); (3) the general wall of the uterus, not related to the embryo, called the decidua vera (_du_). The decidua reflexa and serotina together envelop the chorion, the processes of which fit into crypts in them. At this period both of them are highly and nearly uniformly vascular. The general cavity of the uterus is to a large extent obliterated by the ovum, but still persists as a space filled with mucus, between the decidua reflexa and the decidua vera. [FIG. 151. DIAGRAMMATIC SECTION OF PREGNANT HUMAN UTERUS WITH CONTAINED FOETUS. (From Huxley after Longet.) _al._ allantoic stalk; _nb._ umbilical vesicle; _am._ amnion; _ch._ chorion; _ds._ decidua serotina; _du._ decidua vera; _dr._ decidua reflexa; _l_ Fallopian tube; _c._ cervix uteri; _u._ uterus; z. foetal villi of true placenta; _z´._ villi of non-placental part of chorion.]