THE LATER DEVELOPMENT OF THE CHICK 331 



comes a very considerable embryonic structure, extending 

 finally throughout the neck region. The post-branchial bodies 

 are epithelial buds connected with the fourth visceral pouches, 

 They are to be regarded as vestiges of a fifth pair of visceral 

 pouches and are thus serially homologous with the thymus 

 elements (Fig. 131). 



The thyroid body also arises in connection with this part of 

 the pharynx. This appears during the middle of the second 



TA.4. 



FIG. 131. Derivatives of the visceral pouches and associated organs, in the 

 chick. From Lillie (Development of the Chick). After Verdun (Maurer). 

 Combined from frontal sections. A. In embryo of seven days. B. In embryo 

 of eight days. Ep.3, EpA. Epithelioid bodies derived from third and fourth 

 visceral pouches; J, jugular vein; p'br., p'br. (V)., postbranchial bodies derived 

 from fifth visceral pouch; Ph., pharynx; Th.3., ThA., thymus bodies derived 

 from third and fourth visceral pouches; T'r., thyroid body; ///, IV, third and 

 fourth visceral clefts. 



day as a thickened patch of cells in the floor of the pharynx, 

 between the lower ends of the second visceral 'pouches. This 

 plate of cells evaginates toward the close of the second day 

 (Fig. 123), and soon appears as an entirely closed sac below the 

 pharynx. Later (seventh day) it divides into a pair of vesicles 

 which enlarge and migrate a short distance posteriorly. 



Finally we must describe, as an appendage of the pharynx, 

 the pulmonary tract. For a short distance behind the branchial 

 region the pharynx becomes narrower and is deepened by the 

 formation, late the second day, of a ventral groove (Fig. 130, 



