internal layer, together with the embryo, goes to the bottom. 

 As a result, the embryo, which is still not surrounded by 

 amnion, appears open'. The thin transparent film, easily 

 separated from the embryo, which Wolff called the external 

 layer, is nothing other than the yolk membrane, and the 

 internal layer represents the blastodisc, actually composed 

 of all three embryonic layers. 



To the end of the third day and slightly later, 

 according to Wolff's description, the vascular area also is 

 composed of two layers: upper — thin — and lower — thicker and 

 softer. But at this time they are so closely connected that 

 they appear as one membrane. Near the embryo there is a 

 transparent region; in its range both of these membranes begin 

 to separate and the embryo becomes distinct. It appears 

 between them so that the upper membrane passes above the 

 embryo, and the lower passes under it. 



Interpreting the description from the context of present 

 knowledge about chick development, it can be suggested 

 that the picture of separation of the two "membranes" corres- 

 ponds to the formation of the exocoelom, i.e. the extra- 

 embryonic space, limited by the parietal and visceral layers 

 of the mesoderm, so that Wolff's "upper membrane" is probably 

 the ectoderm together with the parietal layer of the mesoderm, 

 and "the lower membrane" is the visceral layer of the mesoderm 

 together with the endoderm. 



Wolff described the subsequent fate of the lower layer 

 of the vascular area (Figure 7) as follows: 



This layer goes from all sides of the vascular area 

 in the direction of the embryo ... at a higher level , 

 than the embryo and amnion . . . , as if it wants to 

 pass through the amnion lying below with the embryo in 

 it and to cover it. Only a small space remains in the 

 middle of the upper surface of the amnion which is 

 not covered with the internal layer ... After that, 

 when the internal layer reaches this place, it 

 is immediately turned in an acute angle, directed 

 downwards, and lies around the embryo with the thin 

 amnion surrounding it. After this, on the lower surface 



71 



