EXTRA-EMBRYONIC MEMBRANES 



8S 



ectoderm of neural plate 

 ectoderm of blastoderm 



yolk 



primitive pit 



primitive streak 



fore-gut 



neural tube 

 ectoderm of head 

 subcephalic pock 



splanchnopleure 

 of yolk-sac 



open neural groove 

 primitive pit 



B pericardial region 



of coelom 



anterior intestinal 

 portal 



fore-gut 



subcephalic pocket 



extra-embryonic 

 coelom 



dntenor posterior 



intestinal portal intestinal portal 



hind- gut 



sub-caudal pocket 



J- amnion 

 — extra-embryonic 

 coelom 



splanchnopleure 

 of yolk-sac 



pharynx 



post-anal gut 

 proctodaeum 



mid-gut 



allantoic bud 

 yolk- stalk 



Fig. 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. 



