THE FCETAL MEMBRANES OP REPTILES AND BIRDS. 211 



and Birds, it is to be seen that a considerable complication has arisen 

 in the case of the latter. Whereas in Fishes the extra-embryonic 

 area of the somatopleure becomes exclusively the dermal yolk-sac, in 

 Eeptiles and Birds two sacs have arisen out of it by a process of 

 folding. The influences producing this folding appear to be clear. 



Since the egg is enclosed in firmly applied envelopes, the embryonic 

 body, when it is formed by the folding together of the germ-layers, 

 cannot rise from the yolk-sac ; it therefore comes to lie in a depres- 

 sion of the latter. There is the more reason for the occurrence of 

 this because the embryo at the beginning of development is exces- 

 sively small in comparison with the yolk, and because the yolk-layers 

 immediately underlying it become liquefied and absorbed. With 

 the sinking of the body into the yolk (Plate I., figs. 2 and 3), the 

 parts which in Fishes become the simple dermal yolk-sac (Plate I., 

 figs. 6 and 7) fold in around it on all sides as amniotic folds, and 

 enclose it the more completely the deeper it sinks into the yolk. 



The preceding account of the development of the amnion is made some- 

 what schematic in a single point. That is to say, the anterior fold of the 

 amnion is developed so early, that the middle germ-layer has not yet been 

 able to spread out as far as the anterior part of the embryonic area. The in- 

 folding, therefore, in this region involves only the outer and inner germ-layers, 

 which are still closely united. This conditi'on is changed somewhat later, 

 when the middle germ-layer has grown into the region of the anterior fold of 

 the amnion, and has there split into a visceral and a parietal layer. The process 

 has not yet been followed out in detail in series of longitudinal sections. But 

 at all events we must assume that the entoblast, which is united with the 

 visceral middle layer, retracts from the anterior fold of the amnion and 

 again spreads out flat, as is represented in diagrammatic figure 11 (Plate I.). In 

 this manner the anterior amniotic fold, which in the meantime has become 

 greatly enlarged, now consists of the outer germ-layer and the parietal middle 

 layer, as is the case from the beginning with the subsequently arising posterior 

 and lateral folds of the amnion. 



We now have to enter still more particularly upon the further 

 relations of amnion and serosa. 



Up to the end of embryonic development the amniotic sac remains 

 in continuity with a small region on the ventral side of the embryo, 

 which is called the dermal umbilicus. In figs. 3, 4, 5, and 10 

 (Plate I.) this place is indicated by means of a circular line (/in). 

 Here the primitive layers of the body- wall are continuous with the 

 corresponding layers of the amnion, as, for instance, the epidermis of 

 the body with an epithelial layer lining the amniotic cavity. The 

 dermal umbilicus of Reptiles and Birds corresponds therefore with" 



