an important part in preparing the foetal vascularisation of the placenta (compare fig. 12 and 13, with fig. 5, 6, and 18—23). The structure of the subjacent placental pad has some how been simplified. The uterine glands have been destroyed and the remains have nearly disappeared. The empty spaces of the syncytial and trophospongial clusters have augmented in number and extent and can't be distinguished from each other. For this reason the placental pad shows a more spongious and simple structure than in N°. 2a.
The above-mentioned spaces are in connection with the bloodlacunes of the placental cushion especially at the margin of the plane of attachment. On the basal side they penerate within the layer of circular muscles and acquire connection with the venous system of the muscular wall of the uterus. The fig. 12 and 13 are details drawn under high power which illustrate the transition of a vein of this region into the lacunar system of the clusters. In the proximity of the bloodlacunes we may observe numerous leucocytes.
The arterial system on the other hand has a direct connection with the bloodspaces of the trophoderma without intermediate of the lacunes of the clusters. In the fig. 16 and 17 the reader may observe a central artery which is travelling towards the surface of the placental cushion. In the first mentioned figure it is on the point of branching. In the last one it has already divided. One branch remains in the basal layer of the placental pad while the other unites with a large bloodspace in the trophoderma. In the foliowing sections its muscular wTall gradually disappears. The basal branch is also visible in fig. 15 and this branch enters in communication with the arterial system of the muscularis uteri.
Even in this relatively advanced stage there is no direct contact between the maternal and foetal capillary-system, the latter remaining limited to the area vasculosa of the yolksack which nowhere has any connection with the placental region. In the centre of this region, after the disappearance of the embryonic area, the area vasculosa borders the inner wall of the exocoeloma and thus lies close to the placental cushion only separated from it by the narrow exocoelomic space (v. fig. 15). The oxygen and the anabolic materials of the maternal system therefore are obliged to penetrate into the exocoelomic fluid by osmotic interchange and in the same manner these substances will progress from this fluid into the blood circulation of the area vasculosa or into the yolksack before reaching the embryonic region. The catabolic material of this region will progress in inverse direction. After attaining the trophodermal bloodspaces it will be transported at the margin of the placental cushion into the underlying trophospongial lacunar system and finally be removed into the maternal venous system of the muscularis uteri.
III. IST. 9 of the catalogue, diameter of the uterine swelling 7 mM.
Though the gestationsack of N°. 9 is obviously larger than the one of N°. la and the placenta presents a further developmental stage, the embryonic region has lagged behind. A neuroporus is present and the medullary tube has for the greater part not yet closed. The differentiation of the braintube has only just begun. The cephalic flexure is not very obvious and in consequence. the proamnion is still rudimental (v. fig. 18). At the hindside the lateral amnion folds have united with the tailfold (v. fig. 20), but in the anterior trunk region and in the greater part of the headregion no amnionfolds can be detected. A pericard and a straight hearttube are present, but the differentiation of this region is much lower than in N°. la. I have not observed any bloodvessels with undeniable endothelial walls in the embryonic area, but perhaps venae omphalomesentericae are present and one obtains the impression that the vessels of the area vasculosa possess a real wall of their own. The outer wall of the exocoeloma is still devoid of bloodvessels.
Fig. 18 gives a schematic idea of the relative position of the embryonic region, the placenta, the mesometrium etc. The sections are cut in an obliquely transverse direction to the main axis of the embryo i. e. from left-anterior to right-posterior. The embryo shows the same position as the one of N°. 2a. It is laying at the mesometrical side of the germbladder between the two placental pads, the caudal region being directed towards the true placental