. Text-book of embryology. Embryology. VIII EMBRYONIC ADAPTATIONS IN REPTILES 481 sac, more opaque than the allantois and already much smaller than the latter as seen in surface view. The edges of the allantois and the mushroom-shaped yolk-sac fit closely together and between them is the body of the embryo contained in the amnion. As in other Saurop- sidans or Prototherian Mammals the yolk-sac lies on the embryo's left, the allantois upon its right—upon the side, in this case, next the mesometrium. As development proceeds the exposed area of yolk-sac becomes gradually reduced by the encroachme
. Text-book of embryology. Embryology. VIII EMBRYONIC ADAPTATIONS IN REPTILES 481 sac, more opaque than the allantois and already much smaller than the latter as seen in surface view. The edges of the allantois and the mushroom-shaped yolk-sac fit closely together and between them is the body of the embryo contained in the amnion. As in other Saurop- sidans or Prototherian Mammals the yolk-sac lies on the embryo's left, the allantois upon its right—upon the side, in this case, next the mesometrium. As development proceeds the exposed area of yolk-sac becomes gradually reduced by the encroachment of the allantois. The latter however remains merely in contact with the edge of the yolk-sac and never comes to surround it. Over the yolk-sac area there remain visible for a long time the remains of the vitelline mem- brane (of. Bird). Both allantoic and yolk-sac regions of the surface develop placental ar- rangements, the former being physiologically the more impor- tant of the two. The allantoic placenta is already becoming apparent at the stage shown in Pig. 219, A, in the form of an elliptical area at the mesometrial pole which adheres to the uterine lining by means of numerous little pro- jections which interlock with similar projections on a corre- sponding uterine area. As de- velopment goes on the egg assumes an elongated shape (Fig. 219, B). The whole of the uterine lining in contact with the outer surface of the " egg " is provided with a rich ™ lnm in longest dia,„etel. seen from apical Capillary network lying Close pole, showing foetal portion of allantoic placenta (pi). beneath the uterine epithe- lium and here and there insinuating itself between the epithelial cells. Over the allantoic placental area the maternal projections now form undulating ribbons attached along one edge and free along the other. On the surface of these ribbons the uterine epi- thelium instead of being flattened as it is elsewhere is columnar and has a glandular app
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