9 o 



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 SeessePs 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. 



MetencephaJt 



A/lesencephalon % cephalic flexut 

 fypophys 



/lortic arches 

 I.Z.1.4.6 



A/ofochord 



Diencephalon 



Int. carotid artery 



Optic vesicle 



Tros en cephalon 

 Mouth cavity 



Pharyngeal 

 pouches l-*f- 



Ventral aorta. 



Atrium of 

 heart 



UmbilicaJ 

 Vein 



Liver an/age 



Splanchnic 

 tnesoderm 



Entoderm of 



Hind-gut 



Tail 



UmbilicaJ artery 

 Me.'ionephric duct 



,/oaca. 

 I 



Atlanta is 



FIG. 81. 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: (i) 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 



