150 



THE BIOLOGY OF THE FROG 



CHAP. 



Leucocytes are often found between the epithelial cells, 

 and also wandering cells of larger size with bodies of vari- 

 ous kinds in their protoplasm (Heidenhain, De Bruyne). 



rpr^' 



H bl 



Fig. 41. — Pnrtof a cross section of the small intestine of the frog, bl, blood 

 vessels; eg, goblet cells; ep, ordinary epithelial cell; e.s, submucosa; 

 m.c, circular muscles ; m.l, longitudinal muscles ; pe, peritoneum. 

 (After Howes.) 



Wandering cells containing pigment have been found to 

 occur in the lower end of the small intestine (Oppel). 



The mucosa of the small intestine is thrown into numer- 

 ous folds, but there are no true villi nor definite glands nor 

 crypts such as occur in the higher vertebrates. Just behind 

 the pylorus the folds take the form of an irregular network, 

 but a short distance farther back they become arranged in 

 two series of transverse semilunar plications the free edges 

 of which are produced backward, forming a double series 

 of pockets which tend to check the flow of food in the 

 direction of the stomach. The pockets are connected by 

 smaller folds which run mainly in a longitudinal direction. 

 Farther back, a little beyond the middle of the intestine, 



