26 DISCOVERY OF PROTOPLASM. 



continuous coat, and, so to speak, builds itself a little chamber wherein to live. We 

 may therefore distinguish naked protoplasm from that kind which inhabits the 

 interior of a cell of its own creation, and compare the former to a shell-less snail, 

 and the latter to a snail that constructs the house in which its life is spent. Still 

 better may we compare the firm and solid cell-membrane with which the protoplasm 

 clothes itself to a protective coat, a garment fitted to the body; and, following out 

 this analogy, the protoplasm must be designated the living entity in the cell, and 

 the secreted envelope must be considered as merely the skin of the cell. Conse- 

 quently, although this cell-wall was the part which was first revealed by magni- 

 fying glasses, and was called a cell on account of its form, this is not the essential 

 formative element, which has the power of nourishing and reproducing itself. 

 It is the body within the cell, the slimy, colourless protoplasm in full activity within 

 the surrounding membrane made by itself, which must be taken to be the essential 

 part of the cell and the basis of life. 



The term cell had become so naturalized in the science that protoplasm which 

 had escaped from a cell-cavity was also called a cell, and the unfortunate name of 

 " naked cell " was brought into use to designate it. More recently many of these 

 older designations have been abandoned as unsuitable. We now include under 

 the term "protoplasts" all these individual organisms, consisting of protoplasm, 

 which occupy little chambers made by themselves, living either alone like hermits or 

 side by side in sociable alliance in more or less extensive structures, able under 

 certain circumstances to leave their domiciles, laying aside their envelopes and 

 swimming about as naked globules. 



Only when the protoplasts live in innumerable little cavities congregated close 

 together in colonies, and when these cavities are bounded by even walls and are for 

 the most part uniformly developed in all directions, does the part of a plant com- 

 posed of them look under the microscope like a honey-comb, and each cavity like a 

 cell. But even in these cases of external similarity there is the essential diflference 

 that in a honey-comb each of the walls separating individual cells is common to both 

 the adjacent spaces, and, accordingly, the cells of the comb are like excavations in a 

 continuous matrix; whereas, in sections of cellular plants, every cell possesses its own 

 particular and independent wall, so that in them every partition-wall between 

 neighbouring cavities is composed, properly speaking, of two layers (fig. 6). 

 These two layers are scarcely distinguishable in the case of delicate cell-membranes 

 newly secreted by the protoplasts. Later on, however, they are always to be made 

 out clearly (fig. 62). Frequently the layers separate one from another at certain 

 spots, and thus channels are formed between the cells (fig. 6 ^); these are called " inter- 

 cellular spaces." One often sees cells, too, whose entire surfaces are, as it were, 

 glued together with a kind of cement, and then this substance which is stored 

 between the two layers is called "intercellular substance" (fig. 6^). 



By loosening the intercellular substance, where present, by mechanical or chemi- 

 cal means, we can easily separate adjacent cells from one another; the two layers 

 of the partitioning cell- walls come asunder, and then each separate cell exhibits a 



