AMXTPX 



607 



amniotic fold (Fig. 158, A, and 165, 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 (Fig. 165, 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 



FIG. 164. Transverse section across the body of a Chick embryo at about the sixtieth 



hour of incubatin. ( x about 90.) 



oo. aorta ; am. amniotic folds ; am', true amnion, and am", serous membrane ; cd.v. 

 cardinal vein ; ccel'. coelome in body of embryo ; ccet". extra-embryonic coelome ; 

 ect. ectoderm ; end. endoderm ; m. c. medullary cord ; mes'. parietal, and mes!". 

 visceral layer of mesoderm ; m. p. muscle-plate ; ms. d. mesonephric duct ; 

 ms. t. mesonephric tubule ; nc. notochord ; n. cr. neural crest ; r. !. vitelline 

 veins. 



what may be called the embryonic body-wall or somato- 

 plenre (p. 580, Fig. 164, am). Its cavity is a prolonga- 

 tion of the space between the parietal and visceral layers 

 of mesoderm i.e., is an extension of the extra- 

 embryonic coelome. 



The entire amniotic fold gradually closes in above (Fig. 

 165, C) forming a double -layered dome over the embryo. 

 Its inner layer, formed of ectoderm internally and meso- 



