XIII 



AMNION 



607 



amniotic fold (Fig. 158, A, and 165, A, am. /') arises in 

 front of the head end of the embryo frcm 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. /) it consists from the first of ectoderm 

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



am 



am 



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



hour of incubation, (x about 90.) 



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

 cardinal vein ; coeV. ccelome in body of embryo ; coel". 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 ; v. v. vitelline 

 veins. 



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

 pleure (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- 



p P* 



