THE ANATOMY OF A 4.2 MM. HUMAN EMBEYO 



and the pharynx, has broken through so that these cavities are now in communi- 

 cation. The fore-gut, which extends from the oral 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 

 froih the dorsal body wall by the dorsal mesentery. 



Metencephalon. 



Aortic arches 2-4, 6< 



Notochord. 



Descending aorta ' 



Trachea. 



Lung hud, 



Esophagus 



Stomach, 

 Vitelline vein 



Dorsal aorta, 



Hind-gut' 



Mesencephalon and cephalic flexure 

 Hypophysis 



Diencephalon 



Internal carotid artery 



Optic vesicle 



'.I Prosencephalon 

 Mouth cavity 



Pharyngeal pouches 1-4 



Vential aotta 



Atrium of heart 



Umbilical vein 



Liver anlage 



Splanchnic mesoderm 



Mid-gut 



Entoderm of yolk stalk 



Tail i 



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 . 



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

 ticulum of the hypophysis {Rathke's pocket). The fore-gut proper begins with 

 a shallow out-pocketing known as Seessel's pocket. As the pharyngeal mem- 

 brane disappears between these pockets, it would seem that Seessel's pocket 

 represents the persistence of the bhnd 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 phar3Tigeal 



