this spot the muscular wall of the uterus. Finally the groove separates from the cavum uteri, it presents the beginning of the tuba. Tbe right side of the sections therefore agrees with the medial side of the uterus and the placenta lies at the lateral one. Some twenty sections further on the syncytia of -the true placental pad disappear, the uterine mucosa acquires a more or less regular ranged, glandular character. After a few sections we also can detect no more traces of the germbladder.
II. N°. la of the catalogue, diameter of the uterine swelling 6 mM.
Though the size of the uterine swelling N°. la is only slightly larger than that of N°. 2a, the development of germbladder and placenta evidently shows a further stage. Especially in the embryonic area the difference between these two stages is great. In N°. la the neural tube and the amnion-cavity are wholly closed and the encephalon already shows a differentiation in the three primordial vesicles. Optie vesicles are present. The cephalic flexion is obvious and in consequence the head-region projects into the yolksack being covered by a narrow proamnion (v. fig. 14). Pericard and heartrudiment are highly developed and we may observe the principal vessels of the arterial and venous systems (aorta, carotis, venae omphalomesentericae, etc). The foregut has differentiated from the hypoblast which for the rest is spread out flatly and is throughout continuous with the yolksackwall, except in the region of the allantoic bladder, the latter having separated from the umbilical vesicle, communicating with it through a narrow opening. The allantois projects into the exocoeloma without participating in the structure of the placenta.
The area vasculosa shows everywhere numerous bloodvessels with an endothelial wall. As we can observe in fig. 14 the orientation of the embryonic region has greatly changed. The embryo lies no more at the mesometrical side of the uterus, its longitudinal axis being more or less perpendical to the mesometrical plane, but it has shifted to the centre of the germbladder and its main axis has acquired a direction parallel to the mesometrium. On the ventral side (the right one in the drawing) it is covered by the umbilical vesicle, on the dorsal side by the enlarged exocoeloma which occupies nearly half of the germbladder. Thus the back of the embryo nearly touches the placental region, being separated from the latter by the thin amnion only. The mesoblastic covering of the exocoeloma shows no traces of bloodvessels with exception of the spots where it borders the area vasculosa and thus the placenta still is devoid of foetal bloodvessels, since the partly hypoblastic covering of the foregoing stage is wholly pushed away by the mesobiastic one.
In the embryonic region the differentiation of the mesoblast has rapidly progressed. A spacious embryonic coeloma and a large pericardial cavity have arisen, the former being in wide communication with the exocoeloma at the hind part of the embryonic area and ± 18 somites have been formed.
Concerning the structure of the placental cushion and of the two placental pads principal changes have not occurred in it. Especially the false placental pad shows the same features as in the foregoing series. The syncytial clusters however show signs of reduction. The placental cushion has increased in size and projects much further into the exocoeloma. The bloodspaces are much more numerous, the increase being especially evident in the marginal region. Here the maternal trophospongia has been largely reduced. Probably the greater part of the cells have been destroyed and are replaced by trophodermal elements (plasmoditrophoblast). At any rate one can't distinguish with certainty trophospongial elements contributing to the formation of the placental cushion proper and one obtains the impression that it consists of a cortex of cytotrophoblast with radiating branching ingrowths and of bloodspaces with a thin coating of plasmoditrophoblast (compare for the difference between N°. 2a and la fig. 14 and 15 with fig. 4—6),
Remarkable are the shallowness of the central pit and the low development of the other sinuosities of the surface of the placental cushion. In the preceding stage N°. 2a and in the following ones N°. 9 and 10 these hoilow invaginations are much larger and will play