l62 THE ENTODERMAL CANAL AND THE BODY CAVITIES 



embryos the pharyngeal membrane separates the ectoderma mouth 

 cavity, or stomodaum, from the pharyngeal cavity of the fore-gut. In 

 front of the membrane is the ectodermal diverticulum, Rathke's pouch. 

 In 2.5 to 3 mm. embryos (Fig. 167 B) the pharyngeal membrane ruptures 

 and the stomodaeum and pharynx become continuous. The original 

 blind termination of the fore-gut apparently forms Seessel's pouch, a 

 temporary landmark of no special significance. 



The fore-gut later forms part of the oral cavity and is futher dif- 

 ferentiated 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 liver 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 rectum. The cloaca 

 is subdivided into the rectum and urogenital sinus (for its development 

 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 

 connection 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 wholly of in part from the ectoderm. It remains to 

 trace the development of the entodermal pharynx and intestinal tract, 

 and their derivatives. 



THE PHARYNGEAL POUCHES 



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

 entodermal outgrowths 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 

 diverticula. All five pouches come into contact with the ectoderm 

 of corresponding branchial grooves, fuse with it, and form the closing plates. 

 Although the closing plates become perforate in human embryos only 

 occasionally, these pouches, neverthless, are homologous to the functional 

 branchial clefts of fishes and tailed amphibia. The first and second 

 pharyngeal pouches soon connect with the pharyngeal cavity through wide 

 common openings. The third and fourth pouches grow laterad and their 

 diverticula communicate with the pharynx through narrow ducts in 10 to 



