8 4 



HUMAN EMBRYOS AND FETAL MEMBRANES 



form respectively the following adult structures: (i) the auditory tubes; 

 (2) the palatine tonsils; (3) the thymus and parathyreoids ; (4) the.parathy- 

 reoids. Between the pharyngeal pouches are the five branchial arches in 

 which are developed five pairs of aortic arches. Between the bases of 

 the first and second branchial arches, on the floor of the pharynx, is 

 developed the transient tuberculum impar. Posterior to this unpaired 

 structure there grows out ventrally the anlage of the thyreoid gland. From 



Month cavity 



Pharyngeal pouches 1-4 



Esophagus 



Stomach 



Hepatic diverticulu 

 Ventral pancreas 



Mesonephric tubule with glomeruliis 

 Hind-gut 



Allantois 



Thyreoid anlage 



Dorsal pancreas 

 Yolk stalk 

 Mesonephros 

 Mesonephric duct 



:ioaca 



Tail gut 



FIG. 87. Diagrammatic ventral view of pharynx, digestive tube, and mesonephroi of a 

 4-5 mm. embryo (based on reconstructions by Grosser and His). X about 30. The liver and 

 yolk sac are cut away. The tubules of the right mesonephros are shown diagrammatically. 



the caudal end of the trachea have appeared ventrally the lung buds. 

 The trachea is still largely a groove in the ventral wall of the pharynx and 

 esophagus (Fig. 86). Caudal to the lungs, a slight dilation of the digestive 

 tube indicates the position of the stomach. The liver diverticulum has 

 grown out from the fore-gut into the ventral mesentery, cranial to the 

 wall of the yolk stalk. It is much larger than in the fifty-hour chick, 

 where its paired anlage was seen cranial to the intestinal portal, and is 

 separated from the heart by the septum transversum. The small intestine 

 between the liver and yolk stalk is short and broad. In later stages it 



