In 1, 2, 3, 4 the embryo is represented in longitudinal section. 1. Ovum with zona pellucida, blastodermic vesicle, and embryonic area. 2. Ovum with commencing formation of umbilical vesicle and amnion. 3. Ovum with amnion about to close, and commencing allantois. 4. Ovum with villous subzonal membrane, larger allantois, and mouth and anus. 5. Ovum in which the mesoblast of the allantois has extended round the inner surface of the subzonal membrane and united with it to form the chorion. The cavity of the allantois is aborted. This fig. is a diagram of an early human ovum. _d._ zona radiata; _d´._ processes of zona; _sh._ subzonal membrane; _ch._ chorion; _ch.z._ chorionic villi; _am._ amnion; _ks._ head-fold of amnion; _ss._ tail-fold of amnion; _a._ epiblast of embryo; _a´._ epiblast of non-embryonic part of the blastodermic vesicle; _m._ embryonic mesoblast; _m´._ non-embryonic mesoblast; _df._ area vasculosa; _st._ sinus terminalis; _dd._ embryonic hypoblast; _i._ non-embryonic hypoblast; _kh._ cavity of blastodermic vesicle, the greater part of which becomes the cavity of the umbilical vesicle _ds._; _dg._ stalk of umbilical vesicle; _al._ allantois; _e._ embryo; _r._ space between chorion and amnion containing albuminous fluid; _vl._ ventral body wall; _hh._ pericardial cavity.] While the above changes have been taking place the whole blastodermic vesicle, still enclosed in the zona, has become attached to the walls of the uterus. In the case of the typical uterus with two tubular horns, the position of each embryo, when there are several, is marked by a swelling in the walls of the uterus, preparatory to the changes which take place on the formation of the placenta. In the region of each swelling the zona around the blastodermic vesicle is closely embraced, in a ring-like fashion, by the epithelium of the uterine wall. The whole vesicle assumes an oval form, and it lies in the uterus with its two ends free. The embryonic area is placed close to the mesometric attachment of the uterus. In many cases peculiar processes or villi grow out from the ovum (fig. 147, 4, _sz_), which fit into the folds of the uterine epithelium. The nature of these processes requires further elucidation, but in some instances they appear to proceed from the zona (the Rabbit) and in other instances from the subzonal membrane (the Dog). In any case the attachment between the blastodermic vesicle and the uterine wall becomes so close at the time when the body of the embryo is first formed out of the embryonic area, that it is hardly possible to separate them without laceration; and at this period--from the 8th to the 9th day in the Rabbit--it requires the greatest care to remove the ovum from the uterus without injury. It will be understood of course that the attachment above described is at first purely superficial and not vascular.