188 EMBRYOLOGY 



These two lobes differentiate into the thyroid glands, which lose their con- 

 nection with the pharynx and become glands of internal secretion. 



Above the tongue primordia and the thyroid outgrowth five evaginations 

 from the side walls of the foregut form endodermal or visceral pouches. 

 These outgrowths split the mesoderm into visceral arches and make contact 

 with the ectoderm. Some of the pouches fuse with the ectoderm and openings 

 form between the pharynx and the exterior. These openings are called visceral 

 clefts. All these formations are transitory, and the pouches and arches become 

 converted into structures of the head region by processes of migration, fusion, 

 and differentiation. 



The first mesodermal arch forms the basis of the cartilage of the lower 

 jaw and is therefore known as the mandibular arch. The first visceral pouch 

 gives rise to some parts of the middle ear and to the Eustachian tube, which 

 remains connected with the pharynx. The second visceral arch of mesoderm 

 forms the cartilage supporting the tongue — the hyoid cartilage. From the 

 upper parts of the third and fourth endodermal pouches the thymus gland 

 differentiates, while some small glands called epithelial bodies come from 

 the ventral parts of these pouches. Thus, from these outgrowths of the 

 pharynx only the first pouch retains its connection as the Eustachian tube and 

 it functions as a pressure equalizer for the cavity of the middle ear. 



Another outgrowth from the floor of the pharynx forms the lungs and 

 trachea. The early primordium of these structures is a trough of endoderm 

 which splits off from the floor of the pharynx, retaining only its anterior 

 connection. This anterior opening forms the glottis. The free end of the tube 

 splits into two tubes which differentiate into the paired lungs and which 

 communicate with the glottis by means of the trachea. Farther back, the 

 foregut narrows to form the esophagus. 



Concerning the factors which govern the differentiation of the derivatives 

 of the foregut, we can do little but speculate. As a result of our previous 

 studies on the determination of structures in the neurula stage we cannot 

 accept the theory that all these structures are predetermined to form in the 

 precise region in which they occur. There must be inducing factors, therefore, 

 in the surrounding tissues. The surrounding tissues are mesoderm and heart. 

 Perhaps the heart stimulates the endoderm to form the lungs. 



The heart forms very early and is suspended from the floor of the foregut 

 by a mesentery. What is more reasonable than to suppose that various regions 

 of the heart stimulate the various regions of the endoderm above to differen- 

 tiate in a particular fashion? Recall that the roof of the archenteron stimu- 



