72 LECTUEES ON BIOLOGY 



a homogeneous opaque main substance, and embedded in it 

 numerous strongly refractive granules ; higher magnification 

 shows that the refractive granules lie on the fine meshes of a 

 delicate network embedded in the opaque main substance. 

 According to modern opinion we must regard these meshes as 

 the optical transverse section of a cell-system, in other words, 

 we must assume that the cell is built after the manner of a 

 beehive. The walls of the single cells are formed of a more 

 solid substance, whilst their interior is filled with a clear liquid, 

 the cell-juice. But not all cells have such complex organization. 

 In many cases even the most powerful microscope reveals only 

 the hyaline main substance with numberless granules distributed 

 throughout the cell. 



In order to gain an insight into the life-functions of these 

 minute elementary organisms let us examine a simple protozoon, 

 an Amoeba, which we may find at any time in a ditch. At first, 

 frightened by the dazzling light in the microscope, it contracts 

 itself into a sphere, and only the trained eye is able to decide 

 whether the motionless little lump is a living organism or a 

 particle of dirt. But soon the Amoeba adapts itself to the altered 

 circumstances ; it becomes hungry, and with the hunger, active. 

 Slowly, almost cautiously, the protoplasm begins to bulge in one 

 spot and forms a minute pseudopodium. Kemaining undisturbed, 

 the Amoeba gains courage and stretches its little foot further 

 forward; soon other feet follow. At last the little animal is in 

 full activity and moves along with flowing, crawling, lazy 

 movements. 



But in addition to these external amoeboid movements there 

 is the restless motion of the protoplasm itself : the granules move 

 backwards and forwards, here and there, as ants in an ant-heap. 

 This flowing of the granules is characteristic of the protoplasmic 

 movements. It can also be demonstrated in the cells of the 

 higher animals and plants, whose form usually becomes rigid 

 in consequence of the secretion of a firm cell- wall, which prevents 

 the amoeboid motion. It is unnecessary to mention that these 

 movements of the granules are not automatic, but are effected by 

 the flow of the cell-juice. 



This process may be very distinctly observed in protozoa with 

 long pseudopodia, as, for instance, the sun-animalcules (Heliozoa). 



