. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 440 COMPARATIVE ANATOMY Insectivora, and Cheiroptera, though the placenta is also discoidal, it has probably not arisen, like that just mentioned (metadiscoidal form) from a diffuse type, but was originally restricted to a discoidal area, owing to the umbilical vesicle occupying a large surface of the From the above description it is evident that the differences in the form of the placenta are mainly those of degree, and that Ch crion $ 'efa e sse (jhff none/) ithe'L T~.!.. FIG. 334.—DIAGRAM TO ILLUSTRAT
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. 440 COMPARATIVE ANATOMY Insectivora, and Cheiroptera, though the placenta is also discoidal, it has probably not arisen, like that just mentioned (metadiscoidal form) from a diffuse type, but was originally restricted to a discoidal area, owing to the umbilical vesicle occupying a large surface of the From the above description it is evident that the differences in the form of the placenta are mainly those of degree, and that Ch crion $ 'efa e sse (jhff none/) ithe'L T~.!.. FIG. 334.—DIAGRAM TO ILLUSTRATE THE RELATIONS OF THE FCETAL AND MATERNAL VESSELS IN THE HUMAN PLACENTA, SHOWING CHORIONIC AND MATERNAL VESSELS (Gefcisse) AND CAPILLARIES, VILLI (Zotten), AND DECIDUA. (After Keibel.) the placenta gives little indication of the systematic position of the animal in question. In the course of development the embryo becomes more and more folded off from the yolk-sac (Fig. 8), the stalk of which and that of the allantois, enveloped by the base of the amnion, together form the umbilical cord. At birth, the foetal membranes are shed, the intra-abdominal portion of the allantois persisting as a cord, the lira elms, 1 The histological structure of the placenta and the various modifications seen in the maternal mucous membrane cannot be described here ; it is, how- ever, important to remember that there is no direct communication between the maternal and fretal blood, and that the maternal capillaries usually enlarge to form sinuses, the walls of which become invaginated by the villi ; thus the latter are covered by an epithelium furnished by the maternal tissues (Fig. 334). In some cases ( Mole) part of the maternal tissue is absorbed in the uterus,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Wiedershe
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