[26 



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 thyreoid 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- 



Gang.nnJ 



Otccyst 

 Phar.pouchl^ 

 Calif juaalar^ 



Aortic an 



Gana.n.s 



Post hbe hypophysis 



nt. lobe hypophysis 



Phar. pouch 



Caudal root 

 n hypoglossal 



Ganyfrerle/ 



Aortic archl 

 Gany.cervJ 



Phar.pouchlV 



/lorhcarchS 



Phar.pouchl 



Maxillary process 

 Thyreoid aland 

 Pulmonary art. 

 Aorta. 

 YolkJac 

 R. Ventricle 



/£ Atrium 



Stomach 1 



Dorsal pan ere h 



Vitelline arth 



Ventral pancreas \ I 1 l \Umbilical artery 



Uesctnainy Mono- \ \ \ 



\ \ Mesonephrlc duct 



Mothord Cephalic limb, iniesl. loop 



Fig. i 18. — 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). 



