HISTOLOGY OF THE MUCOUS MEMBRANE. 239 



cells, other cells take part in the active ingestion of food, and this 

 again is most marked in the lower Vertebrates, more especially in 

 Fishes and Dipnoans. These are the lymph-cells (leucocytes, 

 which on this account have received the suitable name of phago- 

 cytes), which are present in great numbers in the connective-tissue 

 of the mucous membrane (Fig. 194, A, Ly), often united into 

 definite masses (Z/). These cells, which are capable of the most 

 complete amoeboid movements, become migratory, force their way 

 through the epithelium of the gut (Fig. 194, A, Z 1 , _ 3 ), and come 

 into contact with the food in the lumen of the intestine (L 2 , NN). 

 Others of these phagocytes again, seem to take up the nutritious 

 particles only after the latter have penetrated through the epithelium 

 into the connective-tissue layer. Here they become incorporated 

 by the lymph-trunks (Lyni), and finally, through metabolism, by 

 the whole organism. 



The phagocytes possess the further peculiarity of taking up noxious 

 substances or portions of tissues which have undergone retrogressive meta- 

 morphosis, wherever they may occur in the body, thus rendering them 

 innocuous. They thus exercise a kind of superintendence over the body, 

 acting as a sort of police force. This function is seen most plainly in the 

 migration of leucocytes from the tonsils into the mouth and pharynx, and 

 from the epithelium of the conjunctiva into the conjunctival sac. It is not 

 improbable that this process of ingestion by the leucocytes takes place at every 

 point where the mucous membrane becomes continuous with the outer skin 

 (e.g. nose, urethra, anus). 



Thus we arrive at the result that, in the lower Vertebrates, 

 and, with certain limitations, in the higher types also, active or 

 mechanical processes take place in digestion. These appear 

 to be of great importance in most of the Anamnia; thus in 

 all Fishes and Dipnoi, for instance, glands, provided with 

 specially differentiated epithelial cells, are not present, or at any 

 rate only the first traces of them can be recognised. With the 

 exception of the liver, Amphioxus, Cyclostomi, and 

 Dipnoi possess no trace of glands, and even in Amphibia, 

 a marked differentiation does not seem to occur. It cannot be 

 affirmed that no chemical processes take place in the process of 

 digestion in these animals, for every individual epithelial cell of 

 the gut may be looked upon as a small gland ; but, at all events, 

 the chemical processes in the higher types from the Reptilia 

 onwards, become of far greater importance than the merely mech- 

 anical absorptive processes, owing to the development of highly 

 differentiated glandular organs (peptic glands and glands of 

 Lieberkiihn). 



In conclusion, attention must be directed to the formation of 

 folds of the mucous membrane. In Cyclostomes these have only 

 a longitudinal direction (Fig. 195, A), while, from Elasmobranchs 

 onwards, they take on a transverse arrangement, and thus crypts 

 arise which possess a sac-like form, often passing deeply into the 



