THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 



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 



Metencephalon 



Aortic arches 2-4, 



Notochord. 



Descending aort 



Trachea, 

 Lung bud 

 Esophagus 



Hind-gu, 



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 mesoderm 



Mid-gut 



Entoderm of yolk stalk 



Tail gut 



Umbilical artery 

 Mesonephric duct 



Cloaca 

 Allantoi's 

 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 SeesseVs 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 



