• 1
    The changes in the uterine wall preceding and during the process of implantation of the blastocysts are described. The formation of the uterine crypts results from rearrangement of the cells of the uterine epithelium and they open into the uterine lumen from the time of their inception. This account differs from Hubrecht's (1894), who claimed that the crypts were formed from nests of epithelial cells beneath the epithelium which acquired a secondary opening by the breaking clown of the overlying epithelium.
  • 2
    The changes in the structure and arrangement of the embryonic membranes during and after the attachment of the blastocyst to the uterine wall are described in sequence.
  • 3
    Amniogenesis is by fold formation after the embryonic plate has been exposed by the disappearance of the cells of Rauber. The amniotic and chorionic parts of the folds are distinct from the beginning and there is no transformation of the one into the other. A transient sero-amniotic connection is formed from the cells of the amniotic wall of the folds at the point where they close, far back over the embryo. The proamnion is large and is invaginated, together with the yolk-sac, by the formation of the cervical flexure. The mesoderm only extends into this region at a late stage in development.
  • 4
    The allantoic mesoderm differentiates very early, almost immediately after the formation of the exocoel. The entodermal allantoic diverticulum is formed later, is exceedingly rudimentary and transitory. The allantoic stalk is not provided with a trophoblastic sheath, like that of Crocidura (Sansom, 1937). The allantoic mesoderm fuses with the chorion, forming the allantochorion, very soon after the closure of the amniotic folds, and the adherence of the ehorionic trophoblast to the mucosa.
  • 5
    Vestiges of a decidua capsularis corresponding to that of Erinaceus are found in Sorex, but a complete decidua eapsularis is not formed. Small transitory folds, which are quite distinet from the vestiges of the decidua capsularis, are, found in Sorex araneus and it is suggested that these correspond to the so-called “ decidua eapsularis ” of Crocidura.
  • 6
    Reichert's membrane is well developed in the Shrews. It is formed between the mesoderm and trophoblast of the chorion and between the entoderm and trophoblast of the bilaminar omphalopleure. Therefore it appears to be of purely trophoblastic origin. It disappears in the placental region after the establishment of the allanto-chorion. Later its free margin forms a supporting collar around the base of the allantoic stalk.
  • 7
    The whole of the upper or splanchnic wall of the yolk-sac is invaginated within the lower or bilaminar wall so that the sinus terminalis runs around the rim of the cup so formed. The entodcrm cells of the area vasculosa become enlarged and their cytoplasm charged with granules. The small blood vessels of this region project into the cavity of the yolk-sac, forming villi which are covered with these enlarged entoderm cells and apparently are absorptive organs. The yolk-sac remains large and its bilaminar wall persists throughout development, though its cavity is greatly reduced by invagination and the consequent approximation of its outer and inner walls.