amphioxus and the invertebrates. Fig.110. Ovum of a rabbit from the uterus, one-sixth of an inch in diameter. Fig. 111. The same ovum, seen in profile. Fig. 112. Ovum of a rabbit from the uterus, one-fourth of an inch in diameter. Fig. 113. The same ovum, seen in profile. Fig. 114. Ovum of a rabbit from the uterus, one-third of an inch in diameter. Fig. 110—Ovum of a rabbit from the uterus, one sixth of an inch in diameter. The embryonic vesicle (_b_) has withdrawn a little from the smooth ovolemma (_a_). In the middle of the ovolemma we see the round germinal disk (blastodiscus, _c_), at the edge of which (at _d_) the inner layer of the embryonic vesicle is already beginning to expand. (Figs. 110–114 from _ Bischoff._ Fig. 111—The same ovum, seen in profile. Letters as in Fig. 110. Fig. 112—Ovum of a rabbit from the uterus, one-fourth of an inch in diameter. The blastoderm is already for the most part two-layered (_b_). The ovolemma, or outer envelope, is tufted (_a_). Fig. 113—The same ovum, seen in profile. Letters as in Fig. 112. Fig. 114—Ovum of a rabbit from the uterus, one-third of an inch in diameter. The embryonic vesicle is now nearly everywhere two-layered (_k_) only remaining one-layered below (at _d_). The small, circular, whitish, and opaque spot which the gastric disk (Fig. 106) forms at a certain part of the surface of the clear and transparent embryonic vesicle has long been known to science, and compared to the germinal disk of the birds and reptiles. Sometimes it has been called the germinal disk, sometimes the germinal spot, and usually the germinative area. From the area the further development of the embryo proceeds. However, the larger part of the embryonic vesicle of the mammal is not directly used for building up the later body, but for the construction of the temporary umbilical vesicle. The embryo separates from this in proportion as it grows at its expense; the two are only connected by the yelk-duct (the stalk of the yelk-sac), and this maintains the direct communication between the cavity of the umbilical vesicle and the forming visceral cavity (Fig. 105). Fig.115. Round germinative area of the rabbit. Fig. 116. Oval area, with the opaque whitish border of the dark area without. Fig. 115—Round germinative area of the rabbit, divided into the central light area (_area pellucida_) and the peripheral dark area (_area opaca_). The light area seems darker on account of the dark ground appearing through it. Fig. 116—Oval area, with the opaque whitish border of the dark area without.