UNICELLULAR ORGANISMS. 177 



modifications of structure are beginning to appear. In many cases this 

 differentiation induces a sort of superficial resemblance to more highly 

 specialised organisms, such as worms and other multicellular creatures, 

 which, in opposition to the Protozoa, have been lately called "Metazoa." 

 "We can thus understand how it was that a famous investigator once 

 attempted to interpret the organization of the Infusoria (which are, 

 however, conspicuously Protozoa) entirely in accordance with the 

 analogy of higher, and indeed of the highest animals. 1 



This attempt, indeed, was made at a time when there was not only 

 no knowledge of the structure of the single cell, nor true insight into 

 the histology of the animal body, but when the Infusoria were almost 

 the only Protozoa known. Through the discovery and study of other 

 forms (especially through the labours of Stein, Johannes Mliller, Lieber- 

 klihn, Haeckel, and Hertwig), a belief in a "perfect organization" of 

 the Protozoa has become impossible, for the vast majority of them are 

 destitute of that differentiation which distinguishes the Infusoria, 

 and exhibit characters which stamp them indisputably as unicellular 



anisms. 



If we are rightly to express the morphological nature of the Pro- 

 tozoa, we cannot of course support the old idea of Schwann, according 

 to which the cell was " a hollow nucleated vesicle, surrounded by a 

 transparent structureless wall," in which the contents are of only 

 subordinate importance. While it is true that the cell membrane is, 

 in virtue of its properties of firmness, porosity, &c., of most funda- 

 mental physiological importance in the economy of the cell, yet, on 

 the other hand, it is equally certain thanks especially to the investi- 

 gations and generalisations of Max Schultze that it is by no means 

 the essential part of the cell. It is indeed not to the surrounding 

 membrane, but to the living protoplasm to the formerly undervalued 

 cell-contents that the characteristic capacities of the cell are to be 

 referred. Movement and irritability, assimilation and growth in a 

 word, all the vital phenomena of the organism are dependent on the 

 properties of this protoplasm. 



We cannot retain the slightest doubt on this point when we re- 

 member that there are cells, both among higher and lower organisms, 

 which never become enclosed in a membrane at all, or only at a late stage, 

 but which in their naked membraneless state exhibit the phenomena 

 of life even more conspicuously than the normal membrane-clad cells. 



In these naked cells one can observe, under certain circumstances, 

 and especially in their normal conditions, a peculiar movement. The 

 protoplasm gathers itself at certain convenient points in the form of 

 processes, which grow out under the eye of the observer into lobular or 



8 Ehrenberg, " Die Infusionsthierchen als vollkommne Organismen :" Leipzig, 1838. 



M 



