CHE MOT AXIS. 307 



organisms protect themselves against inimical cells 

 by ingesting, killing, and finally discharging or 

 digesting the latter. 



The botanist, Pfeiffer, first described the phe- 

 nomena of negative and positive chemotaxis in re- 

 lation to the myxomycetes. Under certain condi- 

 tions they either are attracted toward or move 

 from moist places. That a negative chemotaxis 

 may be changed into a positive was shown in rela- 

 tion to salt solutions. When placed in the vicinity 

 of or in contact with strong solutions the cell re- 

 cedes, whereas if one passes gradually from weaker 

 to stronger solutions the latter eventually attract 

 rather than repel the cell. 



As one goes higher in the animal scale intracel- 

 lular digestion for purposes of nutrition is con- 

 fined to rather definite groups of cells. The intes- 

 tinal epithelium of certain invertebrates consists 

 of "sessile phagocytes," cells which, individually 

 or after fusion into plasmodial masses, surround 

 and digest solid particles of food. It is said that 

 in sponges the digestive tract is not sharply sepa- 

 rated from the mesodermal tissue, and the cells of 

 the latter share with the former the function of 

 intracellular digestion. 



In higher invertebrates and in all vertebrates 

 the intestinal epithelium ceases to be essentially 

 phagocytic, digestion being accomplished rather by 

 ferments which have been secreted by the intestinal 

 and related glandular epithelium. Such animals, 

 nevertheless, possess an abundance of phagocytic 

 cells, but they are in the main mesoblastic in na- 

 ture, and may have nothing more than a remote 

 relationship to the nutrition of the organism. 



