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THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



culum 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 two pockets, it would seem that Seessel's pocket 

 represents the persistence of the blind anterior end of the fore-gut. No other 

 significance has been assigned to it. 



Mesencephalon tf cephalic flexure 

 Hypophysis 



MdcncephaJot 



Aortic arches 

 Z.2.3.4.6 



A/otochord 



Diencephalon 



Int. carotid artery 



Optic vesicle 



Trosencephalon 

 MouTh cavity 



Pharyngeal 

 pouches I-**- 



— Ventral aorta. 



Atrium of 

 heart 



Umbilical 



Vein 

 Liver an/age 



Splanchnic 

 mesoderm 



Mid-cjuf- 

 Entoderm of 



yoik-sta/k 



Tail aut 

 Umbilical artery 

 Meson ephrfc duct 



^Cloaca. 



Allantoi 



Fig. 8i. — Diagrammatic reconstruction of a 4.2 mm. human embryo, viewed from the right side (adapted 



from a model by His). 



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

 pouches. Later a fifth pair of pouches is developed (Fig. 82). The four pairs 

 of pharyngeal pouches are important as they form respectively the following 

 adult structures: (1) the Eustachian tubes; (2) the palatine tonsils; (3) the 

 thymus anlages; (4) the parathyreoids or epithelial bodies. Between the pharyn- 

 geal 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 



