The embryo, which was probably not quite normal (fig. 165 A), was very imperfectly developed; a medullary plate was hardly indicated, and, though the mesoblast was unsegmented, the head fold, separating the embryo from the yolk-sack (_um_), was already indicated. The amnion (_am_) was completely formed, and vitelline vessels had made their appearance. [FIG. 165. THREE EARLY HUMAN EMBRYOS. (Copied from His.) A. An early embryo described by His from the side. _am._ amnion; _um._ umbilical vesicle; _ch._ chorion, to which the embryo is attached by a stalk. B. Embryo described by Allen Thomson about 12-14 days. _um._ umbilical vesicle; _md._ medullary groove. C. Young embryo described by His. _um._ umbilical vesicle.] Two embryos described by Allen Thomson (No. 239) are but slightly older than the above embryos of His. Both of them probably belong to the first fortnight of pregnancy. In both cases the embryo was more or less folded off from the yolk-sack, and in one of them the medullary groove was still widely open, except in the region of the neck (fig. 165 B). The allantoic stalk, if present, was not clearly made out, and the condition of the amnion was also not fully studied. The smaller of the two ova was just 6 mm. in its largest diameter, and was nearly completely covered with simple villi, more developed on one side than on the other. In a somewhat later period, about the stage of a chick at the end of the second day, the medullary folds are completely closed, the region of the brain already marked, and the cranial flexure commencing. The mesoblast is divided up into numerous somites, and the mandibular and first two branchial arches are indicated. The embryo is still but incompletely folded off from the yolk-sack below. In a still older stage the cranial flexure becomes still more pronounced, placing the mid-brain at the end of the long axis of the body. The body also begins to be ventrally curved (fig. 165 C). Externally human embryos at this age are characterised by the small size of the anterior end of the head. [FIG. 166. TWO VIEWS OF A HUMAN EMBRYO OF BETWEEN THE THIRD AND FOURTH WEEK. A. Side view. (From Kölliker; after Allen Thomson.) _a._ amnion; _b._ umbilical vesicle; _c._ mandibular arch; _e._ hyoid arch; _f._ commencing anterior limb; _g._ primitive auditory vesicle; _h._ eye; _i._ heart. B. Dorsal view to shew the attachment of the dilated allantoic stalk to the chorion. (From a sketch by Allen Thomson.) _am._ amnion; _all._ allantois; _ys._ yolk-sack.]