HISTOLOGY OF THE MUCOUS MEMBRANE 267 



HISTOLOGY OF THE MUCOUS MEMBRANE OF THE ALIMENTARY 



CANAL. 



The epithelium lining the alimentary canal of Vertebrates with 

 the exception of that of the mouth and cloaca, which is usually 

 stratified consists primitively, that is, phylogenetically, of amoeboid 

 or cilated cells. In some cases this is also true ontogenetically, and 

 in Amphioxus and Protopterus, for instance, the ciliated epithelium 

 persists throughout life, and in the Ammocoete until metamorphosis. 

 In the adult Petromyzon, as well as in many Fishes and even Amphi- 

 bians, ciliated epithelium occurs constantly only in certain parts of 

 the gut, and in the higher Vertebrates cilia are only seen excep- 

 tionally after the embryonic period, so that, as a rule, only ordinary 

 columnar epithelium is present. A striated margin is observable 

 along the free border of the columnar cells (Fig. 214, A, B, a) ; this 

 is probably to be looked upon as the expression of the earlier 

 ciliated covering, and in some lower Vertebrates (e.g., Proteus and 

 Salamander larvae) it is capable of an active amoeboid movement 

 (B, 5). In this active participation of the cells in the process of 

 absorption, we recognise an old inheritance from primitive Inver- 

 tebrates (inlracellular digestion) ; but, at the same time, extra- 

 cellular or chemical digestion is the more important and occurs 

 exclusively in the higher types. 



Numerous lymph-cells or leucocytes (p. 290) are present in great 

 numbers in the connective-tissue layer of the mucous membrane, 

 and often form definite masses or follicles (e.g., "Peyer's patches "). 

 In some cases (e.g., Protopterus), the development of this lymphatic 

 tissue along the gut is relatively enormous (cornp. p. 334). The 

 amoeboid leucocytes may even force their way into the lumen of 

 the intestine : a similar migration of these cells occurs in all 

 mucous membranes and in the walls of many vessels. 



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

 alimentary epithelium must be considered as secretoiy, each 

 individual cell acting as an independent gland. In Fishes, 

 Amphibians, and Reptiles, a higher stage is reached, inasmuch 

 as groups of cells in the stomach gives rise to tubular glands of a 

 simple nature (Fig-. 214). 1 A further differentation of the cells 

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

 which the cells become differentiated into chief cells and parietal cells. 



According to their position, three kinds of glands can be dis- 

 tinguished in the stomach of Mammals, viz., cardiac, fundus-, and 

 pyloric glands. The fundus glands have the greatest physiological 

 importance (comp. Fig. 212). 



In the higher Vertebrates, more especially in Birds and 

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



1 Even in Ganoids and Teleosts the cells of the neck and fundus of each gland 

 are said to be different in character, pepsin being formed in the latter. 



