or the placental pad s. s. since trophoblast and nterine mucosa will grow together at this spot in more central sections to form the placental cushion.
In central direction the exocoeloma rapidly increases in extent. The coelomic space of fig. 2. appears to be the right horn of the exocoeloma. Some 70 sectiones centrad we find another coelomic space before the primitive streak. In reality this ought to be to the left of it, the oblique cutting-direction causing this optie deceit. As the primitive streak disappears some 15 sections centrad, the two horns unite to a large common space, which covers abont a third of the circumference of the germbladder at its bind side.
In the meantime the embryonic area lias disappeared, some traces of the somatic mesoblast still are visible, while the number and extent of syncytial clusters in the true placental pad have increased, the few uterine glands having hypertrophied (especially at their mouth in the uterine cavity). The exocoelomic part of the germbladder lies just opposite to the placental pad and these two structures are approaching each other, as the germbladder grows larger.
Fig. 3 presents a section through this region of the gestationsack. We observe the large germbladder nearly filling the cavum uteri. The true placental pad at the left side is obviously thicker and shows much larger syncytial clusters than the false one on the right side.
The exocoeloma covers the hind part of the germbladder, at the ventral side rudiments of the area vasculosa are seen. The greater part of the anterior wall of the germbladder is formed by hypoblast and trophoblast only, laying close together. At the dorsal side we may observe some traces of the somatic and of the ventral mesoblast,
By the time that the trophoblast membrane which covers the exocoeloma, nearly touches the now very flat uterine epithelium, there appears within the exocoelomic space an island of cells. This soon grows together with the outer wall of the exocoeloma. These cells form part of the placental cushion. The latter has the form of a toadstool the margin of which projects into the exocoeloma of the germbladder. About six sections centrad placental pad and placental cushion grow together and the separating layers of the trophoblast and the uterine epithelium disappear.
Fig. 4 gives us a scheme of a marginal section through the placental cushion when it has already united with the placental pad. On the upperside it is bordered by the thin mesoblastlayer of the exocoeloma. From the periphery inwards follow a layer of cylindrical cytotrophoblastcells and the syncytial layer of the plasmoditrophoblast. The latter shows many outgrowths in the subjacent maternal trophospongia which are engaged in surrounding the maternal bloodspaces with a syncytial pseudendothelium. The plasmoditrophoblast gradually passes into the very loose connective tissue of the maternal trophospongia. (Willey calls it dermatic proliferation, but I can't see the necessity of the introduction of new names). This tissue shows numerous empty spaces which could be presented only partly in the drawing in consequence of their smallness. They do not show bloodcoagulations as do the above mentioned peripheral ones. Since both systems of lacunes are evidently in connection with each other, I don't think this fact is of principal importance. The accumulation of bloodplasma in the periphery of the placental cushion will probably be caused by the contraction of the tissues when fixed. The blood will have been pushed under these circumstances to the spots of least resistance.
Syncytial clusters do not contribute to the structure of the placental cushion proper, but remain limited to the placental pad underneath the latter. They also show several gaps and spaces with or without bloodcoagulum which communicate with the ones in the trophospongia of the placental cushion. The greater part of the uterine glands have disappeared in the placental region. In flg. 4 we remark one normal gland on the left side and in the middle of the drawing a hypertrophied one with degenerated, flat epithelium filled with bloodcoagulum. The last mentioned structure however might present the combination of a trophospongial space with a hypertrophied bloodcapillary.
The connective tissue of the placental pad is much denser than that of the placental