372 SCIEXCE PROGRESS. 



like reserve material, it may be drawn, when required, into 

 the metabolism. For instance the starch granule in the 

 plant cell, the glycogen particle in the animal cell, so long 

 as they remain without chemical change, as reserve material 

 in the protoplasm, are not living. They only become living 

 substances when, decomposed and dissolved, they take part 

 in metabolism. The inner and outer skeletal portions of 

 the cell, and also certain solid intercellular substances such 

 as bone and cartilage cannot be considered as living, in the 

 real sense of the word. 



There can, however, be little doubt that, in many cells, 

 certain parts are in a state of active metabolism in spite of 

 possessing a form so unvarying that it gives the characteristic 

 stamp to the whole cell. Striped and smooth muscle fibre, 

 the ciliated epithelial cell, and the infusorian cell are well- 

 known examples of this. The question then arises : 

 " How is it possible that a fluid mass such as, for instance, a 

 ciliated cell, can have an unvarying and complicated form ? " 

 The more or less rigid connection of the particles which 

 we necessarily associate with solidity cannot be admitted 

 since that would make metabolism impossible. In attempting 

 to solve this question, some have assumed that, in cells 

 which possess an unvarying form, the protoplasm is of a 

 medium consistence, half fluid, half solid, and there is little 

 to be said against this suggestion. The degree of fluidity, 

 i.e., of mobility of the particles, varies greatly in different 

 fluids, and it is therefore possible that, in some cases, living 

 matter has the characteristics of a very thick fluid. On the 

 other hand, a high degree of mobility of the particles is a 

 sine qua non for active metabolism, which not only extends 

 over the surface but penetrates the cell in its entirety. The 

 presence of metabolism in such comparatively rigid forms 

 as a muscle fibril or a ciliated cell implies great activity of 

 the particles, and thus the assumption just mentioned, viz., 

 that of a semi-solid, semi-fluid consistency on the part of 

 the protoplasm is of no real assistance in an attempt to re- 

 concile the apparently contradictory facts of the active 

 movement of the component particles and unvarying form. 



In this case as well as in that of many other physio- 



