_e._ epiblast; _h._ hypoblast; _m´._ amniotic mesoblast; _m´´._ splanchnic mesoblast; _am._ amnion; _ev._ cavity of amnion; _all._ allantois; _f._ rudimentary blastopore; _mc._ cavity of vesicle continuous with body cavity; _mm._ mucous membrane of uterus; _m´m´._ parts where vascular uterine tissue perforates hypoblast of blastodermic vesicle; _vt._ uterine vascular tissue; _l._ limits of uterine tissue.] On this day a cavity develops in the interior of this body which at the same time enlarges itself. The greater part of its wall next attaches itself to the free end of the cylinder, and becomes considerably thickened. The remainder of the wall adjoining the cavity of the cylinder becomes a comparatively thin membrane. At the free end of the cylinder there appears on the thirteenth day an embryonic area similar to that of other Mammalia. It is at first round but soon becomes pyriform, and in it there appear a primitive streak and groove; and on their appearance it becomes obvious _that the outer layer of the cylinder is the hypoblast[95], instead of, as in all other Mammalia, the epiblast; and that the epiblast is formed by the wall of the inner vesicle, i.e. the original solid body placed at the end of the cylinder_. Thus the dorsal surface of the embryo is turned inwards, and the ventral surface outwards, and the ordinary position of the layers is completely inverted. [95] According to Hensen the hypoblast grows round the inside of the wall of the cylinder from the body which he regards as the ovum. The original wall of the cylinder persists as a very thin layer separated from the hypoblast by a membrane. The previously cylindrical egg next assumes a spherical form, and the mesoblast arises in connection with the primitive streak in the manner already described. A splanchnic layer of mesoblast attaches itself to the inner side of the outer hypoblastic wall of the egg, a somatic layer to the epiblast of the inner vesicle, and a mass of mesoblast grows out into the cavity of the larger vesicle forming the commencement of the allantois. The general structure of the ovum at this stage is represented on fig. 162, copied from Schäfer; and the condition of the whole ovum will best be understood by a description of this figure.