DIFFERENTIATION IN THE FOREGUT REGION 



187 



the endoderm gives rise to the cells which secrete the digestive enzymes, as 

 well as those which secrete certain hormones. 



While the various folds are closing off the embryonic digestive system 

 from the yolk sac, differentiation of the individual structures such as lungs, 

 liver, and pancreas proceeds. We shall discuss the formation of some struc- 

 tures which are not a part of the digestive system but arise in close association 

 with it, as, for example, the thyroid and thymus glands. 



Differentiation in the foregut region 



Referring to the diagram of the foregut (Fig. 112), we see that the 

 ectoderm fuses with the endoderm of the anterior ventral region of the 

 foregut and forms an oral plate. This plate becomes perforated and the oral 

 cavity and mouth differentiate. A little farther back the tongue arises as two 

 growths from the floor of the pharynx. These two growths fuse and form 

 the lining and glands of the tongue, while the muscle comes from the nearby 

 mesoderm. Between the two primordia of the tongue a pocket develops. 

 This appears first as a single outgrowth but later it splits into two lobes. 



ESOPHAGUS 



GLOTTIS 

 TRACHEA 



EPITHELIAL 

 BODIES / 2 OUTGROWTHS OF TONGUE 



PAIRED THYROIDS 



Fig. 112. Derivatives of the foregut and some associated structures. The 

 visceral pouches, lettered a through e, are endodermal outgrowths of the foregut. 

 The mesodermal arches are solid black and labeled 1 through 5. Some of the 

 derivatives of the visceral pouches and visceral arches are shown in the figure. 

 Others are described in the text. The thyroid, the lining of tongue and lungs, 

 and the trachea come from the endoderm. The hypophysis is ectodermal in origin 

 and bears no relation to the development of the foregut. 



