PHARYNGEAL POUCHES 169 



gut, the cloacal membrane of the hind-gut. In 2 mm. embryos the pharyngeal 

 membrane separates the ventral ectodermal cavity, or stomodaum, from the 

 pharyngeal cavity of the fore-gut. Cranial to the membrane is the ectodermal 

 diverticulum, Rathke's pocket. In 2.5 to 3 mm. embryos (Fig. 161 B) the pharyn- 

 geal membrane is perforated and the stomodseum and pharynx are continuous. 

 The blind termination of the fore-gut probably forms Seessel's pocket. 



The fore-gut later forms part of the oral cavity and is further differentiated 

 into the pharynx and its derivatives; into the esophagus, respiratory organs, 

 stomach, duodenum, jejunum and a portion of the ileum. From the duodenum 

 arise the liver and pancreas. The hind-gut, beginning at the attachment of the 

 yolk-stalk extends caudally to the cloaca, into which opens the allantois in 2 mm. 

 embryos. The hind-gut is differentiated into the ileum, caecum, colon and rec- 

 tum. The cloaca is subdivided into the rectum and urogenital sinus (for its de- 

 velopment see Chapter VIII). At the same time, the cloacal membrane is 

 separated into a urogenital membrane and into an anal membrane. The latter 

 eventually ruptures and this opening is the anus. The yolk-stalk usually loses 

 its connection with the entodermal tube in embryos of 7.5 mm. (Fig. 172). 



We have seen how the palatine processes divide the primitive oral cavity 

 into the nasal passages and mouth cavity of the adult, and have described the 

 development of the tongue, teeth and salivary glands, organs derived wholly or 

 in part from the ectoderm. It remains to trace the development of the pharynx 

 and its derivatives. 



PHARYNGEAL POUCHES 



There are developed early from the lateral wall of the pharynx paired out- 

 growths which are formed in succession cephalo-caudad. In 4 to 5 mm. embryos, 

 five pairs of pharyngeal pouches are present, the fifth pair being rudimentary. 

 At the same time there appears in the mid- ventral wall of the pharynx, between 

 the first and second branchial arches, a small rounded prominence, the thyreoid 

 anlage. This constricts off and forms a stalked vesicle (Fig. 82). Its stalk, the 

 thyreo-glossal duct, opens near the aboral border of the tuberculum impar. 

 Meantime, the pharynx has been flattened dorso-ventrally and broadened later- 

 ally and cephalad so that it is triangular in ventral view (Figs. 82 and 162). 



From each pharyngeal pouch develop small dorsal and large ventral 

 diverticula. The first four pouches come into contact with the ectoderm of 

 the branchial clefts, fuse with it and form the closing plates. Only occasionally 

 do the closing plates become perforate in human embryos. The first and second 



