102 



TEXT-BOOK OF EMBRYOLOGY. 



from the inner — the amnion — to form a second membrane consisting externally 

 of ectoderm, internally of mesoderm and called at first the serosa or false 

 amnion, later the primitive chorion (Fig. 96). 



3. That the extraembryonic body cavity unites across the medial line 

 dorsally, thus separating the amnion from the primitive chorion (Fig. 97, a, b 

 and c). 



During the formation of the amnion the chick embryo is becoming more and 

 more definitely constricted off from the underlying large yolk mass which is 

 liquefying and into which the embryo sinks somewhat. At the same time the 



Ant. vitelline vein Mesoderm 



Omphalomesenteric 

 (vitelline) vein 



Primitive streak 



Area opaea 

 Sinus terminalis 



Extraembryonic body cavity 



Amnion 

 Amniotic suture 



Area pellucida 



Amniotic suture 

 V Lateral amniotic fold 



Tail amniotic fold 

 Area opaca 



Fig. 94. — Dorsal view of embryo of albatross, showing amnion covering greater 

 part of embryo. Sckauinsland. 



amniotic cavity continues to increase in size and extends also ventrally beneath 

 the embryo so that the embryo is everywhere enclosed within the amnion 

 except at its narrow connection with the yolk (Fig. 97, c, d). 



The amniotic cavity is filled with fluid, the liquor amnii, the origin of which 

 is uncertain. In it the embryo floats freely, attached only by its ventral con- 

 nection with the yolk. At about the fifth day of incubation rhythmical con- 

 tractions of the amnion begin. These are apparently due to the development 

 of contractile fibers in its mesodermic .tissue and give to the embryo a regular 

 oscillating motion. 



