474 Prof. 8. Apdthy 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 Metazoon, 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 impossible. THE VERY FACT OF THEIR 
REMAINING TOGETHER IN A COLONY, AND THEIR INCAPABILITY 
TO LIVE INDEPENDENTLY, ARE SIGNS OF THE INDIVIDUAL 
DEBILITATION OF THE SEVERAL PROTOBLASTS; in consequence 
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, 
however, the endoderm cells become specialized for the prepa- 
ration of digestive juices and ferments, 7. e. the consequence 
of their debilitation is the transformation of their protoplasm 
to digestive juices, and moreover without the application of a 
