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 de^nite 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). 



margin of anterior 

 horn of mesoderm — 



.posterior margin of ; 

 subcephalic pocket — -- 



margin of fore-gut ~j 



margin of anterior 



intestinal portal 



(entoderm) 



notochord 



margm ot area opaca 



ectoderm of head 

 mesenchyme 



41— border of mesoderm 



pericardial region 

 of coelom 



thickened splanchnic 

 mesoderm 



neural fold 



Fig. i6.- 



- Ventral view ( X 37) of cephalic region of chick embryo having 5 pairs 

 of somites (about 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. i6). 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 

 cephaHc 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 hne marks the extent to which the 

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

 bottom of the subcephaHc pocket where the ectoderm of the 

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

 blastoderm. Comparison of Figure i6 with the sagittal section 

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



