474 Prof. S. Apiithy on 



occasionally also extra-cellular. For how could we desig- 

 nate otherwise than as extra-cellular digestion the capacity 

 of certain Bacteria to dissolve by their secretions caoutchouc 

 and other substances which are difficult to assail and to absorb 

 them as nutriment? 



That extra-cellular digestion is of such limited occurrence 

 among the Protozoa, nay even that it can only take place 

 under exceptional circumstances, is solely due to external 

 conditions, which render extra-cellular digestion a physical 

 impossibility for the majority of the Protozoa. Under the 

 term digestion we understand only the process of the con- 

 version of solid nutriment into a solution or into a fine emul- 

 sion. In this the chief part is played by the digestive 

 secretions and ferments. In extra-cellular digestion the food 

 is exposed to the influences of the cell-body externally to the 

 latter ; in intra-cellular digestion, however, this takes place 

 within the body of the cell. Now how should a Protozoon, 

 supposing it to be possible for the digestive juices to be pro- 

 duced at all without immediate stimulation of the proto- 

 plasm, secure their effect upon the food outside its body? 

 The Protozoon must, in order to be able to digest, in order to 

 render possible the operation of the digestive juices upon its 

 food, incorporate its nutriment. But if this can also take 

 place outside the cell, in consequence of the position of the 

 cells in the colony, it will be possible to omit the incorpora- 

 tion of the food into the cells. 



It is therefore in no way wonderful that the change in the 

 mode of life of the former Protozoon, produced by living 

 together with other cells in the consolidated, individualized, 

 and differentiated colony, or in the Metazoou, should entail 

 an alteration in its habits. In the first place it was of great 

 advantage to be able to store up in an intestinal cavity much 

 more nutriment than the several cells were in a position to 

 secure all at once. An intra-cellular digestion was no longer 

 unavoidable ; but it gradually became for the majority of the 

 cells of the body also im])ossible. The very fact of their 



REMAINING TOGETHER IN A COLONY, AND THEIR INCAPABILITY 

 TO LIVE INDEPENDENTLY, ARE SIGNS OF THE INDIVIDUAL 

 DEBILITATION OF THE SEVEIUL PrOTOBLASTS ; in COUSCqUCncC 

 of further exhaustion the majority of the cells, and gradually 

 also the endoderm cells, forfeit the capacity for active, amoeboid 

 changes of form ; in compensation for the rest of the colony, 

 liowever, the endoderm cells become specialized for the prepa- 

 ration of digestive juices and ferments, i. e. the consequonce 

 of their debilitation is the transfurniation of their protophism 

 to digestive juices, and moreover without the application of a 



