. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine . ned to expose foetus; D, infundibulum ofurachus; B, allantoid portion of umbilical cord. other than the monotremes and marsupials have a true allan-toic placenta. The Discoidal Placenta.—This form of placenta is that exist-ing in the rodentia, insectivora, and cheiroptera. The conditionfound in the rabhit is that which has been most studied. Therelation of parts is shown in Fig;. 79. The uterus of the rodent is two-homed; so we find in gen-eral several embryos in each horn in the pregn
. A text-book of comparative physiology for students and practitioners of comparative (veterinary) medicine . ned to expose foetus; D, infundibulum ofurachus; B, allantoid portion of umbilical cord. other than the monotremes and marsupials have a true allan-toic placenta. The Discoidal Placenta.—This form of placenta is that exist-ing in the rodentia, insectivora, and cheiroptera. The conditionfound in the rabhit is that which has been most studied. Therelation of parts is shown in Fig;. 79. The uterus of the rodent is two-homed; so we find in gen-eral several embryos in each horn in the pregnant are functionally independent, each having its own set of 84 COMPARATIVE PHYSIOLOGY. membranes. It will be observed from the figure tbat tbe truevillous cborion is confined to a comparatively small region;tbere is, however, in addition a false cborion witbout villi, butbigbly vascular. This blending of forms of placentation wbicbexist separately in different groups of animals is tbe rabbit at a later stage tbere is considerable interming-ling of foetal and maternal parts,,. i %~Series of diagrams representing the relations of the decidna to the ovum, atdifferent periods, in the human subject. The decidua are dark, the ovum shadedtransversely. In 4 and 5 the chorionic vascular processes are figured (after Dal-ton), 1. Ovum resting on the decidua serotina; 2. Decidua reflexa growing roundth-ovum; 3. Completion of the decidua around the ovum; 4. Villi, growing outall around the chorion; 5. The villi, specially developed at the site of the futureplacenta, having atrophied elsewhere. The Metadiscoidal Placenta.—This type, which, in generalnaked-eye appearances, greatly resembles the former, is foundin man and the apes. The condition of things in man is by nomeans as well understood as in the lower mammals, especiallyin the early stages; so that, while the following account is that REPRODUCTION. 85 usually given in works on embryology, the student ma
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