INTIMATE STRUCTURE OF CELLS. 31 



bold movement, which is manifested by many cells. The 

 protoplasm surrounding contractile vacuoles exhibits it. It is 

 especially the function of muscle-cells, which decrease in 

 length and at the same time increase in thickness. This 

 movement is also said to be accomplished by the flowing of 

 the fluid protoplasm. 



Ciliary movement : The vibratory movement of cilia and the 

 undulatory movements of flagella have been already described. 

 It is a vital protoplasmic movement. 



Cyclosis : In certain vegetable cells currents of liquid and 

 minute particles are observable coursing about in definite 

 channels entirely within the cell. This movement is called 

 cyclosis, or internal cell-circulation, and is perhaps due to 

 spaces and channels in the cell-substance, along which the con- 

 tained fluid is forced by contractions of the protoplasm. 



Brownian movement: Fine microscopic particles of solid 

 matter, if mixed with water or certain slightly viscid fluids, 

 exhibit under the microscope a characteristic constant dancing 

 or oscillating movement. This was first described in 1826, 

 by Robert Brown, from whom it has been called the Brownian 

 movement ; it is also called pedesis. Its cause is not under- 

 stood. It is a purely physical phenomenon, not vital ; but 

 granules within cells sometimes exhibit this movement. 



Intimate structure of cells : The description of cells above 

 given covers the features observable by means of the micro- 

 scope. When we contemplate how from a single microscopic 

 cell, the ovum, a large organism can develop, composed of 

 millions of cells of a highly differentiated and vastly compli- 

 cated structure ; and how ova indistinguishable from one 

 another, though belonging to different species, produce their 

 own kind with unerring fidelity : we are forced to conclude that 

 the germ-cells must contain elements or mechanisms corre- 

 sponding to and capable of developing all the various parts of 

 the full grown organism that is, that they have a vastly com- 

 plex structure that is hidden from us beyond the limits of micro- 

 scopic vision, and that what we see are but the coarsest feat- 

 ures of their construction. Structural elements not ordinarily 

 visible become manifest during karvokinesis. Between the 

 atomic and molecular structure of which we gain some idea 

 through chemical science, and the elements visible to us, 



