l6o THE ENTODERMAL CANAL AND THE BODY CAVITIES 



fore-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. 167 B) the 

 pharyngeal membrane ruptures and the stomodaeum and pharynx become 

 continuous. The blind termination of the fore-gut apparently forms Seessel's 

 pocket. 



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

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

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

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

 yolk stalk extends caudally to the cloaca, into which the allantois opens 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, forming the anus. The yolk stalk usually loses its connec- 

 tion with the entodermal tube in embryos of about 7 mm. (Fig. 179). 



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 whoUy or 

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

 and the intestinal tract and their 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 such pharyngeal pouches are present, the fifth pair being rudimentary 

 (Figs. 86 and 87). Meantime, the pharynx has been flattened dorso-ventrally 

 and broadened laterally and cephalad, so that it is triangular in ventral view 

 (Figs. 87 and 168). 



From each pharyngeal pouch develop small dorsal and large ventral diver- 

 ticula. All five 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 pharyngeal 

 pouches soon connect with the pharyngeal cavity through wide openings. The 

 third and fourth pouches grow laterad and their diverticula communicate with 



