126 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



outgrowth, caudal to the fourth pouch, is regarded as a fifth pharyngeal pouch 

 in human embryos and forms the post-branchial body on each side (see p. 172). 

 The thy reoid gland, composed of branched cellular cords, is located in the mid- 

 line between the second and third branchial arches (Fig. 118). 



Trachea and Lungs. Caudal to the fourth pharyngeal pouches the eso- 



Ganq.nn-'J %B 



Otbiysf 

 Phar pouch J, 



Ganq.n.s 



Aortic 

 Phar. pouch 



Stomach 

 Dorsal pancre 



Vitelliite ortby 



Venlraf /yahcrea 



Septum 

 trannfenum 



Cloaca. 

 Allantofs 

 Rectum 



Ureter 

 'ef&rtepftros 

 Umbilical artery 

 Mesonephric duel 



Cephalic limb, intest. loop 



FIG. 118. Reconstruction of a 10 mm. pig to show the position of the various organs from the right 

 side. The veins are not indicated. Broken lines indicate the outline of the left mesonephros and the 

 positions of the limb buds. X 10. 



phagus and trachea separate and form entodermal tubes (Figs. 117 and 118). Be- 

 fore the trachea bifurcates to form the primary bronchi there appears on its right 

 side the tracheal bud of the upper lobe of the right lung. This bronchial bud 

 is developed only on the right side and appears in embryos of 8 to 9 mm. Two 

 secondary bronchial buds arise from the primary bronchus of each lung, and form 

 the anlages of the symmetrical lobes of each lung (Fig. 119). 



