their basal parts these spaces penetrate into the muscularis uteri and obtain connection with the maternal venous system (v. fig. 19).
IV. N°. 10 of the catalogue, diameter of the uterine swelling 8 mM.
This series has been damaged seriously in cutting. In consequence. a part of the sections are badly deformed, especially the embryo proper and the embryonic coverings. Happily the placenta has remained intact for the greater part, but the whole gestation sack has been stretched parallel to the mesometrical plane and has been flattened perpendicular to the latter. The cutting-direction is obliquely horizontal in relation to the main axis of the embryo and the orientation of the latter is more are less the same as in the series N°. la. It is lying in the centre of germbladder with its right upperside close to the surface of the placenta] cushion and its longitudinal axis is parallel to the mesometrical plane. The tail has grown out and describes a semi-circle under the ventral region of the body, the end lying on the right side of the head-region.
Tn consequence of this circumstance we will meet thrice with the neural canal in fig. 22, once in the head-region (mind the optie vesicle), a second time in the posterior trunc-region and a third time in the tail.
Embryo and placenta are much higher developed than in the preceding series. On account of the damaged state of the embryo I will not enter in details upon the structure of the former, but I will only mention the facts that four visceral pouches are visible, the first two possessing an outer aperture, that the auditory vesicle has differentiated into sacculus and utriculus and that a epiblastic lenticular thickening is present at the outer side of the optical vesicle. The vascular system is already highly developed and the exocoelomic mesoblast shows numerous large vessels communicating with those of the embryonic area. Numerous somites and nephridial rudiments are present.
Concerning the plaeentation of this stage I only will make a few remarks. The false placental pad has been reduced and is only slightly thicker than the remaining part of the uterine mucosa. The placental cushion has strongly increased in size and is nearly twice as large as in the preceding stages. Its apical covering is still formed by the outer mesoblastic layer of the exocoeloma and is filled up with smaller and larger bloodvessels which enter into the spacious pits and into the cytotrophoblastic villi of the placenta (vide figg. 22 and 23).
The cortex of the placental cushion thus consists of two intermingled reticula of bloodspaces, the foetal capillary system penetrating in centripetal direction, the maternal bloodlacunarsystem progressing in centrifuga] direction, the latter being [covered [by a pseudoendothelium of plasmoditrophoblast. The two systems may easily be distinguished from each other, the maternal one being filled with bloodcoagulum and showing no bloodcorpuscules with exception of some leucocytes, the foetal one lacking bloodplasma but showing numerous nucleated haematoblasts
Fig. 24 presents a tangential section through the margin of the placental cushion. The maternal bloodspaces are presented by a black tone and are surrounded by a plasmoditrophoblastic layer characterized in the drawing by black nuclei and by the absence of celllimits. The cytotrophoblastic covering of the foetal vessels which in opposition to the maternal bloodspaces possess a wall of their own and show numerous haematoblasts, is indicated by the dotted condition of the nuclei and by the presence of cell-limits. At the periphery we may observe the foetal vessels of the exocoelomic wall penetrating into the trophoderma of the placental cushion.
In the figg. 22 and 23 we may obtain a general idea of the structure of the centre of the placental region. We may observe the numerous large pits penetrating irom the surface to the basal parts of the cushion being filled up partly by branching foetal bloodvessels. These vessels belong to the vascular system of the exocoelomic mesoblastic wall, the allantois not yet participating in the structure of the placenta. In the peripheral layer the