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 ™ 16 . 16 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 appearance. With the ribbon-like projections 

 just mentioned there interlock the somewhat similar projections of 

 the foetus. These are also covered with columnar epithelium close 

 under which lies a rich capillary network. The latter is not confined 

 VOL. II 2 I 



Fig. 219. — " Egg" of Chalcides tridactylus. 

 (After Giacomiui, 1891.) 



A, 7 min. in diameter, showing yolk-sac (y.s), allan- 

 tois (all), and foetal portion of allantoic placenta (pi) ; 



