pad. If the headregion should dive into the yolksack and the amnion eavity be closed, the position of the embryo in relation to the exocoeloma and the placental region would be just the inverse of that in the series N°. la, i. e. the ventral side of the embryonic region would be turned towards the placenta and the exocoeloma and the dorsal side towards the yolksack.
Concerning the embryonic region we may remark that the foregut has differentiated from the yolksack, the remaining part of the gut wall being still continuous with the coating of the umbilical vesicle. Some 8 somites are visible, but in consequence of the tranversal cutting-direction the number can't be determined exactly. Besides the pericard with the heartrudiment, paired embryonic coelomasacks are present, the right one (the left one of the sections) being in connection with the exocoeloma. At the end of the segmented region of the trunc the right (left of the sections) amnionfold appears, ± 40 sections backwards the left one becomes visible. In the meantime the medullar tube flattens to a broad plate and grows together with the mesoblastic layer which has become solid, forming together the primitive streak and the two lateral amnionfolds unite with the tailfold. In consequence of the oblique cutting direction this junction takes place in an asymmetrical way viz. the right fold (being at the same time the caudal one) progressing much faster than the left one. As result of this process a continuous amnion now covers the primitive streak and the left lateral coelomic space unites with the .exocoeloma above the amnion (vid. fig. 20).
An entodermal allantois still is wanting. Perhaps a mesoblastic thickening at the right (hind) side of the tailfold which is visible in nearly 30 sections, presents a rudiment of a mesoblastic allantoic outgrowth. Anyhow there is no question of this allantoic rudiment exercising any influence upon the formation of the placenta as we many conclude from the circonstance that series N°. la with an evident and relatively large entoblastic allantois has a less developed placenta than series N°. 9 which is devoid of an entoblastic allantoisrndiment.
The false placental pad of this stage is characterized by very spacious uterine glands with a thin wall and by a reduction of the syncytial clusters. It gradually takes a more ore less puerperal appearance. In the true placental pad on the other hand nearly the whole mucosal part is syncytial and shows large, anastomosing empty spaces which communicate on the basal side with the maternal venous system and on the apical side with the bloodspaces of the trophoderma. In the placental cushion the matei-nel trophospongial part has almost disappeared especially in central sections. The trophodermal ingrowths have largely increased and penetrate in the centre into the underlying syncytial tissue of the placental pad. The large anastomosing trophodermal bloodlacunes are throughout surrounded by a pseudendothelium of plasmoditrophoblast which is fully separated from the cytotrophoblastic villi by narrow empty spaces (v. fig 21). These villi are partly bollow and these small cavities will in future obtain connection with the wide and deep peripheral pits which characterize this stage of the plaeentation and the following in opposition to the foregoing one (compare figg. 14 and 15 with figg. 18 and 19). The mesoblastic covering of the placenta does not coat these pits in this stage but forms bridges across their mouth (vide figg. 18—21). This hollowing of the cytotrophoblastic villi will prepare the ingrowth of the foetal bloodvessels of the exocoelomic mesoblast into the trophoderma a process occurring in the following stage. In series N°. 9 the outer wall of the exocoeloma being still devoid of foetal vessels there can be no question of this phenomenon.
The circulation of the maternal blood in the placental lacunar system of this stage takes place in the manner already previously described. Maternal arteriae of the muscularis uteri penetrate through the syncytial tissue of the placental pad towards the centre of the trophodermic cone (v. figg. 19 and 21) and enter in communication with the trophodermal bloodlacunes. On the other hand these spaces are in connection with the vacuities of the syncytial tissue in the placental pad especially at the margin of the placental cushion. In