206 THE ENDODERMAL DERIVATIVES 



tubular midgut becomes very much elongated into a doubly-coiled 

 tube which may be nine to ten times as long as the body of the tad- 

 pole. During metamorphosis, and while the tadpole changes from an 

 herbivorous (of the tadpole) to an omnivorous (of the frog) diet, this 

 midgut (potential small intestine) shortens to about three times the 

 length of the body, and its histology changes correspondingly. That 

 portion of the midgut directly posterior to the stomach becomes 

 the bent duodenum or duodenal loop. The small intestine, like the 

 stomach, is lined with a glandular mucosa and is supplied with both 

 circular and longitudinal (involuntary and smooth) muscles and an 

 outer serosa. It is suspended within the body cavity by a thin but 

 double layer of the peritoneal epithelium. 



During the the earliest stages of development (2.5 mm. stage) there 

 may be seen a sub-notochordal or hypochordal rod of pigmented cells, 

 two or three cells in diameter, lying between the roof of the midgut 

 and the notochord. There is positional and structural evidence that 

 this column of cells is at some time associated with the roof of the 

 archenteron, from the level of the liver to the posteriorly placed 

 neurenteric canal. It becomes entirely free from gut and notochord by 

 the 4.5 mm. stage and disappears shortly after the time of hatching. 

 It has no known function. It may be of evolutionary and ontogenetic 

 significance only. 



The Hindgut 



This is the smallest portion of the original archenteron which lies 

 posterior to the yolk mass, between it and the posterior body wall. 

 The endoderm ventral to the closed blastopore evaginates to fuse 

 with the invaginating proctodeal ectoderm to form the anal plate. 

 This plate finally ruptures at about the 4 mm. stage to form the anus, 

 only the inner portion of which is lined with endoderm. The rectum 

 is the enlarged posterior end of the archenteron, and is therefore 

 endodermally lined. This does not develop until the time of metamor- 

 phosis. 



Dorsal to the rectum are a temporary extension of the archenteron, 

 developed in consequence of the presence of the neurenteric canal, 

 and the posterior growth of the notochord. This is the post-anal gut. 

 At the 5 mm. stage only a remnant of the neurenteric canal can be 

 identified as parallel rows of pigmented cells extending ventro-poste- 

 riorly from the hindgut. This is the region of the disappearing post- 



