EXTRA-EMBRYONIC MEMBRANES 



toderm of neural plate 



primitive pit 



primitive streak 



ectoderm of blastoderm 



fore-gut 



neural tube 

 ectoderm of head 

 subcephalic pocket 



stomodaeum 

 Rathke's pocket 



D 



anterior intestinal 

 . portal 



llantoic bud 

 yolk- stalk 



PIG. 31. Schematic longitudinal-section diagrams of the chick showing, 

 four stages in the formation of the gut tract. The embryos are represented as 

 unaffected by torsion. 



A, chick toward the end of the first day of incubation; no regional differentia- 

 tion of primitive gut is as yet apparent. B, toward the end of the second day;, 

 fore-gut established. C, chick of about three days; fore-gut, mid-gut and hind- 

 gut established. D, chick of about four days; fore-gut and hind-gut increased 

 in length at expense of mid-gut; yolk-stalk formed. 



