shines through their fine-grained protoplasm (Fig. 108). At one part (_c_) inside this hollow ball we find a circular disc, formed of darker, softer, and rounder cells, the dark-grained entodermic cells (Fig. 109). [25] In man and the other mammals the fertilisation of the ova probably takes place, as a rule, in the oviduct; here the ova, which issue from the female ovary in the shape of the Graafian follicle, and enter the inner aperture of the oviduct, encounter the mobile sperm-cells of the male seed, which pass into the uterus at copulation, and from this into the external aperture of the oviduct. Impregnation rarely takes place in the ovary or in the womb. Fig.108. Four entodermic cells from the vesicle of the rabbit. Fig. 109. Two entodermic cells from the embryonic vesicle of the rabbit. Fig. 108—Four entodermic cells from the embryonic vesicle of the rabbit. Fig. 109—Two entodermic cells from the embryonic vesicle of the rabbit. The characteristic embryonic form that the developing mammal now exhibits has up to the present usually been called the “blastula” (Bischoff), “sac-shaped embryo” (Baer), “vesicular embryo” (_vesicula blastodermica,_ or, briefly, _blastosphæra_). The wall of the hollow vesicle, which consists of a single layer of cells, was called the “blastoderm,” and was supposed to be equivalent to the cell-layer of the same name that forms the wall of the real blastula of the amphioxus and many of the invertebrates (such as _Monoxenia,_ Fig. 29 _F, G_). Formerly this real blastula was generally believed to be equivalent to the embryonic vesicle of the mammal. However, this is by no means the case. What is called the “blastula” of the mammal and the real blastula of the amphioxus and many of the invertebrates are totally different embryonic structures. The latter (blastula) is palingenetic, and precedes the formation of the gastrula. The former (blastodermic vesicle) is cenogenetic, and follows gastrulation. The globular wall of the blastula is a real blastoderm, and consists of homogeneous (blastodermic) cells; it is not yet differentiated into the two primary germinal layers. But the globular wall of the mammal vesicle is the differentiated ectoderm, and at one point in it we find a circular disk of quite different cells—the entoderm. The round cavity, filled with fluid, inside the real blastula is the segmentation-cavity. But the similar cavity within the mammal vesicle is the yelk-sac cavity, which is connected with the incipient gut-cavity. This primitive gut-cavity passes directly into the segmentation-cavity in the mammals, in consequence of the peculiar cenogenetic changes in their gastrulation, which we have considered previously (Chapter IX). For these reasons it is very necessary to recognise the secondary embryonic vesicle in the mammal (_gastrocystis_ or _blastocystis_) as a characteristic structure peculiar to this class, and distinguish it carefully from the primary blastula of the