THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 



8.3 



cavity to the yolk stalk, is differentiated into pharynx, thyreoid, trachea 

 and lungs, esophagus and stomach, small intestine and digestive glands 

 (pancreas and liver). The gut is suspended from the dorsal body wall 

 by the dorsal mesentery. 



The ectodermal limits of the oral cavity are indicated dorsad by the 

 diverticulum of the hypophysis (Rathke's pouch). The fore-gut proper 



Metencrpliiilnn 



Aortic arches 2-4, 6- 



Notochord. 



Descending aorta 



Trachea 



Lung bud, 



Esophagus 



Hind-gut 



Mesencephalon and cephalic flexure 

 Rathke's pouch 

 Diencephalon 



Internal carotid artery 



Optic vesicle 

 Prosencephalon 

 Mouth cavity 



Pharyngeal pouches 1-4 



Ventral aorta 

 Atrium of heart 

 Umbilical vein 

 Liver anlage 

 Splanchnic tnesoderm 

 Mid-gut 

 Entoderm of yolk stalk 



Tail gut 



Umbilical artery 

 Mesonephric duct 



Cloaca 



Allantois 



FIG. 86. Diagrammatic reconstruction of a 4.2 mm. human embryo, viewed from the right 

 side (adapted from a model by His). X 25. 



begins with a shallow out-pocketing known as Seessel's pouch. As the 

 pharyngeal membrane disappears between these pockets, it would seem 

 that Seessel's pouch represents the persistence of the blind anterior end 

 of the fore-gut. No other significance has been assigned to it. 



The pharynx is widened laterally, and at this stage shows four 

 pharyngeal pouches (Fig. 87). Later a fifth pair of pouches is developed 

 (Fig. 1 68). The four pairs of pharyngeal pouches are important as they 



