28 THE FROG 



recognized by the oval mass (either clear or dark according 

 to the method of staining) at its outer end. 



(2) The submucosa, a layer consisting mainly of con- 

 nective tissue (characterized by occasional small rounded 

 cells and a large amount of loose fibrous intercellular sub- 

 stance), follows the folds of the mucosa. Within the sub- 

 mucosa are occasionally found large irregular lymph spaces. 



(3) The circular layer of non-striated muscle. This 

 consists of slender spindle-shaped fibers each with an elon- 

 gated nucleus. 



(4) The longitudinal layer of non-striated muscle. 

 The fibers have the same structure as those of the circular 

 layer, but are cut transversely. Surrounding the bundles 

 of muscle fibers, and forming a thin layer outside them, is a 

 small amount of connective tissue. 



(5) The peritoneum or serous layer consists of a single 

 layer of flattened cells cut transversely so that each cell ap- 

 pears spindle-shaped in the section. This layer forms the 

 outer covering of the alimentary canal. 



(b) To show the finer structure draw only a small part 

 of the section under high power. Select for this purpose a 

 portion of the wall of the intestine including a small fold, 

 and showing the above characteristic features as clearly as 

 possible. 



2. Cross-section of the stomach. In a transverse sec- 

 tion taken through the cardiac region of the stomach, iden- 

 tify and study the following layers, which are named in 

 their order from within outward. Study each layer first 

 with the low power, then with the high power. 



(a) The mucosa. This is the innermost layer of the 

 stomach and is so folded as to form long tubular gastric 

 glands set very closely together. 



