THE CELL. 15 



i. Movement of the Naked Protoplasm. 



Probably our most common and typical form of naked proto- 

 plasm is presented to us by the fresh-water ama'ba, found in stagnant 

 water. The amoeba is a. unicellular organism, about Viooo inch in 

 diameter, possessing one or more nuclei, and is almost continually in 

 motion, due to its extending numerous protoplasmic projections, 

 called pseudopodia (false feet). It then rolls fts entire mass into 

 the pseudopodium, or fingerlike projection, only to continue the same 

 operation repeatedly during its life. 



The pseudopodia assume different forms and shapes in the differ- 

 ent kinds of cells, and in this way the establishment of the identity 

 of a cell is frequently aided by an observation of the processes. For 

 example, most of the fresh-water amceba possess broad, lobate or 

 finger-shaped pseudopodia; leucocytes, white blood-corpuscles, 

 divided and pointed pseudopodia ; some of the rhizopods and pigment- 

 cells, threadlike and reticular pseudopodia which flow into one 

 another. 



In the human body some of the cells such as white blood- 

 corpuscles, lymph-corpuscles, and connective-tissue cells possess 

 movements, which, because of their likeness to those of the amceba, 

 are called amoeboid. 



2. Ciliary Movement. 



There have been discovered cells and unicellular organisms 

 possessing delicate, hairlike processes, which extend in greater or less 

 numbers from their surfaces. They are called flagella, or cilia. 

 These resemble very thin pseudopodia when they are composed of 

 hyaloplasm alone, as the cilia and flagella are homogeneous and 

 nongranular in nature. However, they differ from pseudopodia in 

 that their movements are very energetic and always definite, and 

 also that, unlike pseudopodia, their structures are not temporary, 

 but permanent, being neither protruded nor withdrawn. The ciliary 

 cells lining the trachea are subjects for examination. The deep 

 back part of the throat of a frog is gently scraped and the scrapings 

 placed in a drop of water upon a warm stage. When we examine 

 the cells under the microscope, we see upon their surface a constant 

 rapid movement; but the movement is so rapid that we see only the 

 motion, and not the vibrating cilia. If, however, the vibrations be 

 lowered to about a dozen per second, we are then able to see the 

 cilia themselves. Ciliary movements are of various kinds. More 



