xii AMNION AND ALLANTOIS 597 



takes no direct share in the formation of the embryo 

 (Fig. 165). The extension of the ectoderm and endoderm 

 takes place regularly and symmetrically, but the meso- 

 derm, while extending equally in the lateral and posterior 

 regions, grows forwards in the form of paired prolongations 

 which afterwards unite, so that for a time thesis an area of 

 the blastoderm in front of the head of the embryo formed 

 of ectoderm and endoderm only, and called the pro-amnion 



(Fig- *$5,pr- am \ 



Before the embryo has begun to be folded off from the 

 yolk the rudiment of one of the two embryonic membranes, 

 the amnion, has appeared. A crescentic amniotic fold (Fig. 

 164, A, am. f) arises in front of the head-end of the embryo 

 from the region of the pro-amnion / it consists at first of 

 ectoderm only, the mesoderm not having yet spread into the 

 pro-amnion. The fold is soon continued backwards along 

 the sides of the body (B) and round the tail (A), but in 

 these regions (am. f) it consists from the first of ectoderm 

 plus the parietal layer of mesoderm, i.e., it is a fold of what 

 may be called the embryonic body-wall or somatopleure 

 (Fig. 158, so). Its cavity is a prolongation of the space 

 between the parietal and visceral layers of mesoderm, 

 i.e. is an extension of the extra-embryonic ccelome. 



The entire amniotic fold gradually closes in above (c), 

 forming a double layered dome over the embryo. Its inner 

 layer, formed of ectoderm internally and mesoderm exter- 

 nally is the amnion (am\ the cavity of which becomes filled 

 with a watery amniotic fluid, serving as a protective water- 

 cushion to the enclosed embryo. Its outer layer, formed of 

 ectoderm externally and mesoderm internally, is the serous 

 membrane (sr.m) : this comes to lie just beneath the vitelline 

 membrane, with which it subsequently fuses. 



The second of the embryonic membranes, the allantois, 



