HISTOLOGY OF THE MUCOUS MEMBRANE 345 



recognised (intracellular digestion) ; but, at the same time, extra- 

 cellular digestion, in which no external change in the individual 

 cells can be seen, is always the more important in Vertebrates 

 and occurs exclusively in the higher types. 



In Amphioxus, Cyclostomi, and Dipnoi, the whole of the 

 alimentary epithelium must be looked upon as secretory, each 

 individual cell acting as an independent gland. In other Fishes 

 and in Amphibians and Reptiles, a higher stage is reached, inas- 

 much as groups of cells in the stomach give rise to tubular glands of 

 a simple nature. A further differentiation of the cells gradually 

 leads to the condition seen in the gastric glands of Mammals, in 

 which three kinds of glands can be distinguished, viz. cardiac, 



E 



m 



IB 



FIG. 253. SEMIDIAGRAMMATIC FIGCRES OF THE Mucous MEMBRANE OF THE 

 INTESTINE OF FISHES, SHOWING INTERMEDIATE FORMS BETWEEN LONGITU- 

 DINAL FOLDS AND ROUND CRYPTS. (After Edinger.) 



A, Petromyzon, showing the spiral fold ; B, an Elasmobranch ; C to E, various 



Teleosts. 



fundus, and pyloric glands, and in the fundus glands, which have 

 the greatest physiological importance, the cells become differenti- 

 ated into chief cells and parietal cells. 



In the higher Vertebrates, more especially in Birds and 

 Mammals, the epithelium of the intestine also gives rise to tubular 

 intestinal glands (crypts or glands of Lieberkilhii) as well as 

 (in Mammals) to Brunner's glands in the duodenum, closely 

 connected phylogenetically with the pyloric glands of the stomach. 

 Mucus-secreting goblet cells are common throughout the alimentary 

 epithelium of Vertebrates, and the same is true of leucocytes : the 

 latter are especially abundant in the submucosa, from whence they 

 may wander into the lumen of the gut. The lymphoid tissue is 

 often aggregated into definite smaller or larger masses or follicles 

 (e.g. Peyers patches), and in some cases (e.g. Protopterus) is very 

 abundant. 



In order to effect an increase of the absorptive surface, longi- 



