46 EARLY EMBRYOLOGY OF THE CHICK 



neural folds will be seen less and less closely approximated to 

 each other. 



The Establishment of the Fore-gut. In the outgrowth of the 

 head, the entoderm as well as the ectoderm has been involved. 

 As a result the entoderm forms a pocket within the ectoderm, 

 much like a small glove finger within a larger. This entodermic 

 pocket, or fore-gut, is the first part of the digestive tract to ac- 

 quire a definite cellular floor. That part of the gut caudal to the 

 fore-gut where the yolk still constitutes the only floor, is termed 

 the mid-gut. The opening from the mid-gut into the fore-gut 

 is called the anterior intestinal portal (fovea cardiaca). 



proamnion _V;M&' ^N&:-'-.. marg ' n f "'* paCa 



-ectoderm of heat 



/- ; ^^^^Kt^rt r**^ mesenchyme 



posterior margin of '% 



subcephalic pocket -4f ... 



;,';v border of mesoderm 



margin of fore-gut j^',',-' ' ;.' ;'?;' .j*2j&^ pericardial region 



of coelom 



margin of anterior >-:,;. 4mf 



intestinal portal 'I!* YE '"'"' thlckencd s P'anchnic 



(entoderm) -Ijjj W| 



notochord ^~ ^^"l ^ " neural fold 







PIG. 1 6. Ventral view ( X 37) of cephalic region of chick embryo having 5 pairs 

 of somites (abo.ut 25-26 hours of incubation). 



The topography of the fore-gut region at this stage can be 

 made out very well by studying the ventral aspect of entire 

 embryos. The margin of the anterior intestinal portal appears 

 as a well defined crescentic line (Fig. 16). The lateral boun- 

 daries of the fore-gut can be seen to join the caudally directed 

 tips of the crescentic margin of the portal. Considerably 

 cephalic to the intestinal portal an irregularly recurved line can 

 be made out. On either side it appears to merge with the ecto- 

 derm of the head. This line marks the extent to which the 

 head is free from the blastoderm. It is due to the fold at the 

 bottom of the subcephalic pocket where the ectoderm of the 

 under surface of the head is continuous with the ectoderm of the 

 blastoderm. Comparison of Figure 16 with the sagittal section 

 diagrammed in Figure 17, E, will aid in making clear the rela- 



