. Outlines of zoology. disc of formative epibiast;jSw., blastodermic vesicle;H., hypobast. of cells. Development begins in theoviducts, but the eggs are in noway attached to the wall. They arelaid in a nest by the Duckmole; in the Echidna they arehatched in a slight, periodically developed, external pouch. CONNECTION BETWEEN EMBRYO AND MOTHER. 727 {b) In the Marsupials the embryo is born prematurelyafter a short gestation. It is very small and helpless. Tillrecently it was believed that during its intra-uterine life itwas either not attached to the wall of the uterus at all, oronly to a slight
. Outlines of zoology. disc of formative epibiast;jSw., blastodermic vesicle;H., hypobast. of cells. Development begins in theoviducts, but the eggs are in noway attached to the wall. They arelaid in a nest by the Duckmole; in the Echidna they arehatched in a slight, periodically developed, external pouch. CONNECTION BETWEEN EMBRYO AND MOTHER. 727 {b) In the Marsupials the embryo is born prematurelyafter a short gestation. It is very small and helpless. Tillrecently it was believed that during its intra-uterine life itwas either not attached to the wall of the uterus at all, oronly to a slight extent by a yolk-sac placenta. It is nowknown, however, that, in Perameks at least, there is notonly an efficient yolk-sac placenta, but a distinct, thoughsmall, allantoic placenta. The general absence of a. als Fig. 397.—Embryo of Peratiieles with its fetal membranes.—After Hill. am.^ True amnion; al,, allantois; a/.j., allantoic stalk; ,cavity of yolk-sac; ck.^ chorion or false amnion; , sinusterminalis ; ^.c, extra-embryonic body cavity ; z/.t?., vascularomphalopleura, or area of non-separation between yolk-sac walland chorion, constituting the yolk-sac placenta. The endodermis dotted throughout. Note the large size of the yolk-sac, andthe sinking of the embryo into it. placenta in Marsupials, and the small size of the allantois,must therefore be ascribed to degeneration, and not to aprimitive condition. The presence of a yolk-sac placentain Marsupials is not in itself of great importance, for aconnection between the yolk-sac of the embryo and the?wall of the oviduct exists in two Elasmobranch fishes and intwo lizards, but the similarity between the allantoic placentaof Perameks and that of the Eutheria seems to point in-disputably to a common origin for, the two structure
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
